biological development essay

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Theories of Biological Development

Development is a central biological process, and ideas about its nature have been influential in biological thought. This entry surveys the history of these ideas through the lens of “epigenesis vs. preformation”. Epigenesis is, roughly, the thesis that every developing entity starts from material that is unformed, with form emerging gradually, over time, in the process of development. Preformation, in contrast, is the thesis that development begins with the entity in some way already preformed, or predelineated, or predetermined. The question “epigenesis or preformation?” is in part metaphysical: what is it that exists—form or also the unformed that becomes the formed? And it is partly epistemological: how do we know—through observation or inference? Debate on these entangled questions has persisted since ancient times, and today plays out as genetic determinists appeal to the already “formed” through genetic inheritance, while others insist on the efficacy of environmental plasticity. This entry surveys the main theories of development that engage this debate, from Aristotle on generation, to recent systems-theoretic and stem cell-based views. Of course, “epigenesis vs. preformation” is not the only longstanding theoretical opposition that bears on development. But this framing is an inclusive way to capture patterns of transformation and constancy in debates about biological development. Nature or nurture, epigenesis or preformation, genetic determinism or developmental free will, or is some version of a middle ground possible? The terms of this perennial discussion, and the underlying assumptions, continue to shape debates about when life begins and have profound bioethical and policy implications.

1. A Caveat About Theories

2. the problem, 3. aristotle and aristotelianism, 4. eighteenth-century debates, 5. evolution and embryos: a new preformationism, 6. late nineteenth-century debates: weismann and hertwig, 7. late nineteenth-century debates: roux and driesch, 8.1 organicism, gradients, and fields, 8.2 turing’s reaction-diffusion model, 8.3 positional information, 9.1 waddington’s landscape, 9.2 the genetic program, 9.3 the regulatory genome, 10.1 developmental systems and cycles, 10.2 physical and dynamic systems, 10.3 stem cells and lineages, other internet resources, related entries.

A theory of development should explain the core phenomena of growth, cell differentiation, and morphogenesis, which together transform an egg into a mature organism. However, beyond this rough consensus uncertainty abounds. Exactly what entities undergo development, and what are the process’ temporal, spatial, and functional boundaries? These questions are debated in biology and philosophy (Bonner 1974; Pradeu et al. 2011). Also unsettled are the nature and significance of scientific theories. Whether a general theory of development is possible or desirable hinges in part on how “theory” is defined. Yet there is no philosophical consensus to fall back on. Furthermore, developmental biology is not a traditionally theoretical field (in contrast to, say, evolutionary biology; see the entry on developmental biology ). A recent edited collection, Towards a Theory of Development , reveals a variety of viewpoints and frameworks, ranging from skepticism about any such theory, to specific proposals influenced by physics, mathematics, biochemistry, systems biology, and more (Minelli & Pradeu 2014a). Broadly speaking, however,

most developmental biologists have not been strongly interested in constructing an overarching theory of development and… developmental biology has not been a focal topic for philosophers of biology. (Burian 2014: xii)

The subject has received little focused attention in biology or philosophy.

The term “theory”, for the purpose of this entry, refers to broad conceptual frameworks about the nature of the developmental process as a whole. This is more inclusive and less demanding than traditional philosophical notions of theory; “big picture” views offering an overall characterization of and/or basic principles for understanding development. Epigenesis and preformation are alternative theories in this sense: two persistent ways of describing and seeking to explain the development of individual organic form.

The core question underlying the existence of these two competing philosophical traditions is the extent to which something is formed or organized from “the beginning” or whether organization and form arise only over time. Nineteenth-century uses of the term “evolution” included a sense of unfolding of preexisting form, a sort of preformationism in contrast to the epigenesis of the day. Discussions of “evolution” and “epigenesis” can therefore be misleading in retrospect since the former term has assumed a meaning closer to the older meaning of epigenesis (Bowler 1975). Making things even more complicated, by the late nineteenth century “preformationism” really was more about various versions of predetermination or predelineation than preexistence of form as such. Furthermore some authors saw epigenesis or preformation as entirely internally directed, while others in each case allowed responses to the environment. In the background lie debates about the relative significance of predestination and free will, for persons, for organic beings, or even for the inorganic. In each case, it is important to keep in mind what the particular writer was saying and the arguments presented. Thus, discussions of epigenesis and preformation often bring in other ancillary questions and are difficult to separate from their contexts. This entry is an effort to extract what is centrally at issue and to focus on key contributions to the discussion.

The terms of debate can in principle apply to the inorganic world; e.g., solar systems can “evolve” and could develop epigenetically. Yet they are not thought of as doing so, typically, and so epigenesis and preformation are primarily applied to the organic world. Species can evolve or develop more or less gradually, with more or less form already physically existing or programmed in from the beginning. Yet typically discussions of epigenesis and preformation have focused on individual organisms and their development rather than on species. The emphasis is on different interpretations of the actual developmental process as it plays out in time, in individual organisms. Therefore, this entry focuses on individual organisms and understandings of their development processes. Even more specifically, this means looking at the development of their form and to a lesser extent also function. Epigenesis and preformation offer two competing interpretations of what is involved, with a range of alternatives in between. The two approaches draw on different metaphysical and different epistemological sets of assumptions. We can get at the central issues by looking most closely at a series of focused episodes.

Aristotle was a keen observer of many things, including embryos. Looking at chicks, for example, and drawing on his interpretations developed earlier for the physical sciences, he saw material, final, formal, and efficient causes at work in the generation of individual organisms. The early egg was not formed; it did not already contain a little chick (or whatever the species). Instead, an individual animal only gradually acquired a heart that began beating; likewise for the other parts that make it a chick. The material may be there from the beginning, but the formal cause only gradually plays out along with the efficient cause of embryonic development.

Thus, Aristotle could fit his observations of embryos into his larger theoretical interpretations of the world. In sexual generation, individual organisms begin when the fluids from the mother and the father come together. This combines the essential causes and initiates the developmental process. The maternal contribution is the material cause, which resides in the menstrual blood. After “the discharge is over and most of it has passed off, then what remains begins to take shape as a fetus” (Aristotle De Generatione Animalium [GA], 1.727b17–18). Yet the menstrual blood, or female semen, is only that out of which it generates and must be acted upon by the male semen which is that which generates. Together, consistent with the essential nature of the species and telos (or final cause) in question, the formal cause and efficient causal process act to bring a formed individual organism from potential into actual being. Gradually, over time, an individual organism’s form begins to emerge from the unformed. The male and female parents serve as the “principles of generation” for each individual organism (in sexually-generating species).

For Aristotle, an individual life begins when the male and female semen are brought together. This is an external action, which starts the individual formative process. From that point on, the process is internal, initially driven by causes internal to the combined fluids. The process then leads to formation of the individual’s type, since “once a thing has been formed, it must of necessity grow” (Aristotle GA 1.735a18–19). (See Lennox 2001 for discussion of Aristotle’s predecessors on these points; see Van Speybroeck et al. 2002; Vinci & Robert 2005; Connell 2016, 2020; Falcon & Lefebvre 2017; Henry 2017; Hopwood et al. 2018; on Aristotle’s theory of generation and subsequent scientific development of these ideas.)

An individual organic life requires an internal source of motion, or “soul”, which resides in the material body from the outset. This soul guides the gradual epigenetic process of development. This is the Aristotelian and not the Christian soul. Soul consists of the vegetative (for all organisms), locomotory (for animals), and rational (for humans). The soul, consisting of one to three parts depending on the kind of living being, resides in the combination of male and female semen. The living differs from the dead because of the action of the soul. Therefore, it is the teleological drive of the potential that actualizes the individual and its form and function, epigenetically, gradually, and internally.

Aristotelians followed Aristotle and without much further study of embryos interpreted the process of generation, including human development, as gradual and epigenetic. Traditional Catholicism agreed. St. Augustine and St. Thomas Aquinas both held that hominization, or the coming into being of the human, occurs only gradually. Quickening was thought to occur around 40 days, and to be the point at which the merely animal mix of material fluids was ensouled. Until 1859, when Pope Pius IX decreed that life begins at “conception”, the Church was epigenetic along with the Aristotelians (see Maienschein 2003).

Shirley Roe’s discussion of the eighteenth-century debates is an excellent examination of the context. Enthusiasm for scientific study of natural phenomena of all sorts was combined with particular interest in natural history and changes over time and with newly available microscopic methods to stimulate interest in development (Roe 1981). Aristotelian epigenesis still provided the background assumptions about individual development as the eighteenth century began, and researchers sought to observe the gradual emergence of form from non-form. Yet Aristotelian accounts called for the efficacy of the causes, acting through the process of ensoulment. In effect, this interpretation of epigenesis depended on a life force within the organism driving its emergence of form. Those who accepted epigenesis also accepted a form of vitalism (see Detlefsen 2006, Zammito 2018 for more on early modern vitalism debates).

Matter in motion, by itself, would not seem to have the capacity to produce these results. How could unformed matter become formed? How could the emerging form acquire capacity to function without some vital force or factor that was not strictly material? This was the problem for materialists. Those who began with a materialist metaphysics, assuming that all that can exist is matter in motion, could not see how gradual epigenetic emergence of form could occur. The early modern period brought debates between those who started from metaphysical assumptions of materialism and those who started from epistemological assumptions that empirical observation should provide the basis for scientific knowledge.

One popular representation of the alternative, preformationist view was the homunculus. Whether initially intended seriously or as a way to capture alternative ideas, the idea of a tiny preformed little person did capture attention. Nicolaas von Hartsoeker gave us the image in 1694 of a tiny man in the sperm, which became the starting point for spermists. Others, ovists, accepted the idea of a preformed organism—but in the egg rather than sperm. Both views are preformationism taken quite literally. The form that the individual adult organism would assume was, physically and materially, preformed from the earliest stages of development. The subsequent process was just growth.

Not all preformationists took their preformationism quite so literally or graphically. But that view did present a competing alternative to Aristotelian epigenesis by the eighteenth century (see Bowler 1971, Pinto-Correia 1997). As Iris Fry has argued in her study of origins of life debates, only with preformation could a materialistic mechanist be a good Christian in the eighteenth century (Fry 2000: 26, citing Farley 1977: 29). This debate played out, for example, in the work of Caspar Friedrich Wolff and Charles Bonnet, both looking at chick development (Detlefsen 2006). They looked at the same thing and even fundamentally agreed about what they saw, but their conclusions were quite different. This story can be seen as a debate about scientific theory. Wolff was an epigenesist, maintaining that form emerges only gradually. Bonnet was a preformationist, insisting that form exists from the beginning of each individual organism and only experiences growth over time. In addition to these theoretical and epistemological issues, there is also a story about metaphysics. The eighteenth century brought debates between metaphysical materialists who were forced into preformationism, and epistemological epigenesists who observed form emerging only gradually and who were willing to accept vitalism as the only apparent causal explanation for the emergence of form from the not-formed (Roe 1981; Maienschein 2000; Nicoglou & Wolfe 2018 [and references therein]; Wolfe 2021).

In 1859, Darwin focused on embryos and their usefulness for understanding evolutionary relationships. Ernst Haeckel brought the study of embryos to popular attention. And histologists and embryologists, especially in Germany and the United States, used rapidly improving microscopic techniques to observe far more than had been possible before. These observations, in the context of evolutionary interpretations, raised new questions and provoked new answers. The understanding of both epigenesis and preformation underwent transformation so that the debates brought new questions along with the traditional differences.

Darwin pointed to embryology as fundamental for interpreting historical relationships. In Chapter 13 of the Origin he asked

How, then, can we explain these several facts in embryology,—namely the very general, but not universal difference in structure between the embryo and the adult;—of parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;—of embryos of different species within the same class, generally, but not universally, resembling each other;—of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself;—of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed. (1859: 442–443)

But we know that this was a rhetorical question, and sure enough he concluded that

I believe that all these facts can be explained, as follows, on the view of descent with modification. (1859: 443)

And that furthermore,

the leading facts in embryology, which are second in importance to none in natural history, are explained on the principle of slight modifications not appearing, in the many descendants from some one ancient progenitor, at a very early period in the life of each, though perhaps caused at the earliest, and being inherited at a corresponding not early period. Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals. (Darwin 1859: 450)

Haeckel saw ontogeny as the brief and rapid recapitulation of phylogeny and saw each individual’s development as following the sequence of, and indeed caused by, the evolutionary history of that individual organism’s species. In his highly popular books, widely translated and widely read, Haeckel offered pictures of comparative embryology. See, he seemed to suggest, the human form emerges following the evolutionary development and adaptations of its ancestors. Form comes from form of the ancestors, and it unfolds following pre-scripted stages (Haeckel 1866).

Darwin was not an embryologist, and he did not contribute to our understanding of embryogenesis as such. Nor did Haeckel, really. But while Darwin’s use of the embryo in supporting evolutionary theory and in helping to interpret evolutionary relationships was consistent with various versions of either epigenetic or preformationist development, Haeckel’s view was decidedly preformationist. Here, then, was a preformationist interpretation based not on additional embryological observations but on adherence to Haeckel’s own metaphysical view—monistic materialism—and to his desire to provide evidence for evolution. The Haeckelian approach reflected the context in which those studying cells and embryos worked at the end of the nineteenth century.

In 1899, American biologist William Morton Wheeler suggested that there are just two different kinds of thinkers. Some see change and process, while others see stability. Heraclitus, Aristotle, physiology, and epigenesis characterize one way of looking at the world, while Parmenides, Plato, morphology, and preformationism characterizes another. These are, Wheeler felt, stable and persistent classes, with just the nature and details of their differences changing over time. Yet by the end of the nineteenth century, he argued, neither a strict preformationist nor a strict epigeneticist should prevail. Rather he called for a middle ground, for:

The pronounced “epigenecist” of to-day who postulates little or no pre-determination in the germ must gird himself to perform Herculean labors in explaining how the complex heterogeneity of the adult organism can arise from chemical enzymes, while the pronounced “preformationist” of to-day is bound to elucidate the elaborate morphological structure which he insists must be present in the germ.

Furthermore, it is not to philosophy but to science that we must look to resolve the relative contributions of each, for “Both tendencies will find their correctives in investigation” (Wheeler 1899: 284).

Wheeler was stimulated by recent late nineteenth-century debates, themselves provoked by a flood of new discoveries. August Weismann and Oscar Hertwig provided particularly strong and contrasting positions. Weismann had begun from an epigenetic viewpoint and initially rejected the idea that individual form emerges through the unfolding, or evolution, of pre-existent form in the inherited germ. But by the time his Das Keimplasm appeared in 1892 (translated into English in 1893), Weismann had changed his mind. As Weismann wrote:

My doubts as to the validity of Darwin’s theory were for a long time not confined to this point alone: the assumption of the existence of preformed constituents of all parts of the body seemed to me far too easy a solution of the difficult, besides entailing an impossibility in the shape of an absolutely inconceivable aggregation of primary constituents. I therefore endeavoured to see if it were not possible to imagine that the germ-plasm, though of complex structure, was not composed of such an immense number of particles, and that its further complication arose subsequently in the course of development. In other words, what I sought was a substance from which the whole organism might arise by epigenesis, and not by evolution. After repeated attempts in which I more than once imagined myself successful, but all of which broke down when further tested by facts. I finally became convinced that an epigenetic development is an impossibility. Moreover, I found an actual proof of the reality of evolution, which … is so simple that I can scarcely understand how it was possible that it should have escaped my notice so long. (Weismann 1892 [1893: xiii–xiv])

His “proof” provided an account of how, within the context of cell theory and given that the entire body begins in one fertilized cell, all the diverse body parts can become so diversely differentiated. The key is in the special material of the germ cells, Weismann decided. Within these cells lies all the determinants necessary to direct development. Inheritance, that is, causes development and differentiation.

Weismann’s theory postulated the existence of several levels of hypothetical units. By the 1890s, it was agreed that individuals begin as cells, those cells contain nuclei, and that nuclei contain chromosomes. The chromosomes are the material of heredity, Weismann postulated, and they consist of a string of determinants, correlated with characters in the organism. Each determinant consisted of a number of material particles called biophores, inherited from both parents. These biophores compete with each other and some prevail, which then determines the character of the determinant, which in turn determines the character of the organism. During each cell division, the original whole chromosomal material is divided up, so that the effect is like a mosaic. Each cell becomes the right type just because of the action of the determinants distributed to it. As Weismann put it,

Ontogeny, or the development of the individual, depends therefore on a series of gradual qualitative changes in the nuclear substance of the egg-cell.

Cells are self-differentiating

that is to say, the fate of the cells is determined by forces situated within them, and not by external influences. (Weismann 1892 [1893: 32, 134])

Conditions external to the cell itself cannot guide development, but rather the causes lie within. And cell differentiations that make up complex organisms are predetermined. Frederick Churchill’s magnificent biography of Weismann discusses these ideas in depth (Churchill 2015).

Oscar Hertwig disagreed. He felt that Weismann made too many assumptions and actually provided no real explanation of development and differentiation at all. In his work of 1894, Präformation oder Epigenese , Hertwig complained that Weismann’s theory:

merely transfers to an invisible region the solution of a problem that we are trying to solve, at least partially, by investigation of visible characters; and in the invisible region it is impossible to apply the methods of science. So, by its very nature, it is barren to investigation, as there is no means by which investigation may be put to the proof. In this respect it is like its predecessor, the theory of preformation of the eighteenth century. (Hertwig 1894 [1900: 140])

In contrast to Weismann’s preformationism, Hertwig pointed to the interactions of cells and to the differences among cells for the source of differentiation. Complexity is not built in from the beginning, but emerges over time, dynamically, and interactively. A cytologist himself, Hertwig saw the intricate structures already part of the unfertilized egg, and the changes that occur with fertilization. The egg is not a completely unstructured blob, but rather a complex of different materials that can respond to influences both within the egg and from the external environment. Cells behave like small organisms, and it is the interactions of these separate organisms that makes the whole. As Hertwig put it:

I shall explain the gradual, progressive organization of the whole organism as due to the influences upon each other of these numerous elementary organisms in each stage of the development. I cannot regard the development of any creature as a mosaic work. I hold that all the parts develop in connection with each other, the development of each part always being dependent upon the development of the whole.

Furthermore,

during the course of development, there are forces external to the cells that bid them assume the individual characters appropriate to their individual relations to the whole; the determining forces are not within the cells, as the doctrine of determinants supposed. (Hertwig 1894 [1900: 105–106, 138])

Hertwig and Weismann continued to argue, as did others, both about the metaphysical nature of the organism as well as about the epistemological demands for gaining knowledge about it, with no generally accepted way to resolve the issues. Given the information at hand, it seemed that Wheeler was right. There were just two different types of people, drawing on two different sets of values and emphases. Both relied on assumptions, and only new evidence could move the discussion forward.

Wilhelm Roux adopted much the same approach as Weismann’s and so, at first, did Hans Driesch. Yet their experiments ultimately led to new approaches and revised interpretations of what was at issue with epigenesist and preformationist accounts of development (Maienschein 1991b and Weber 2022). In 1888, Roux published results of his experiments on frog eggs. Working on the assumption of a mosaic type preformationism, Roux was persuaded that starting from the very first cell division, each cell would be different because it was already predetermined to be different.

Roux proposed an experiment, a simple and elegant experiment on the face of it. He proposed to take a developing frog egg, after the first cell division, and to separate the two cells. Finding it impossible to separate the two cells, however, he simply killed one by inserting a hot needle. That cell just hung there like a blob of material and no longer differentiated. The other half organism, or single cell proceeded to develop, in Roux’s interpretation, as it normally would have developed (Roux 1888). The half became a half, just as it should if it were already preformed or predetermined as to its fate in the organism. Roux had, it seemed, confirmed the mosaic hypothesis.

A few years later, Driesch was working at Naples and had access to sea urchin eggs. Fortunately, because of Oscar and his brother Richard Hertwig’s study of these eggs, Driesch knew that if he shook the two cells they would separate completely. Driesch reported agreeing with Roux and intending to confirm Roux’s results. But since the sea urchin eggs could actually be separated, he felt that his results would be even more convincing. Imagine his surprise when he looked the next morning after separating the eggs and found not two half embryos but two smaller sized urchin larvae. As he noted,

I must confess that the idea of a free-swimming hemisphere or a half gastrula open lengthwise seemed rather extraordinary. I thought the formations would probably die.

Instead, the next morning I found in their respective dishes typical, actively swimming blastulae of half size. (Driesch 1892 [1964: 46])

In later experiments they developed even further, into apparently perfectly normal pluteus larvae, and even the four cell stage could do the same.

Driesch concluded with an epigenetic account, but an epigenesis relying strictly on materialistic factors (at least that was his initial response; Driesch did turn later to a version of vitalism). The early embryo retains its totipotency, he concluded. The fertilized egg clearly has the capacity to become a whole organism and so, apparently, do the cells after the early cell divisions. Not a mosaic of cells already predetermined by their inherited determinants in the nucleus, the early embryonic stages are instead a population of separate totipotent organisms, each capable of becoming a whole. It is only the interactions among them under normal conditions that lead to a complex, organized, integrated differentiated organism.

It might seem that Roux would have had to acknowledge the superiority of Driesch’s approach, since Driesch had actually separated the cells. But no. Instead Roux countered with an additional hypothesis. The nucleus retains the capacity to adapt, especially in simpler organisms. They need the capacity to regenerate when injured, and therefore the mosaic determination simply has not occurred yet. Each cell retains a “reserve idioplasm”, he argued, and this provides the necessary backup determination needed to form a whole organism (see Churchill 1966, 1973).

It seems that Wheeler was right. Roux, Weismann, and others had decided that development must be guided by predetermined mosaic differences. Preformation, stability, and predictability stood on one side, with epigenesis, dynamic process, and change on the other. And, as Wheeler noted, by 1899 the way forward lay between the extremes of strict preformation or epigenesis. Wheeler’s dissertation director Charles Otis Whitman agreed. Whitman felt that what biology needed was a clear statement of the alternative views, and then movement to a new standpoint examining how much depends on the organism’s developmental response to external conditions drawing on preformation, rather than on programmed internal unfolding alone.

Whitman, Edmund Beecher Wilson, and others at the Marine Biological Laboratory in Woods Hole, Massachusetts, dedicated considerable energy to discovering the nature of each cell and its internal organization and relationships, in an attempt to discover the relative contributions of preformation and epigenetic development to a materialistic explanation of development. By the early twentieth century, they had moved toward an understanding that included a fertilized egg that was to some extent preorganized and differentiated, including in the nuclear chromosomes, and also a capacity of the individual organism to respond to changes in its environment or to self-regulate. This was epigenesis allowing some minimal predeterminism.

8. Regulative Theories of Development

This “epigenesis-first” view of development set the stage for a strand of theorizing that continued well into the twentieth century. The main theoretical tension in twentieth- and twenty-first-century studies of development is not between classic preformation and epigenesis, but between different views about control of developmental processes. If genes are the primary source of control, then organismal development is preformationist in an important sense: it is genetically determined. But if control is more distributed or holistic, such that developing organisms are (in some sense) self-organizing, then the process is regulated in a way recalling classic theories of epigenesis. Several influential theories of morphogenesis and pattern formation take the latter view. Gene-based accounts of development take the former. In this way, the classic theoretical opposition between preformation and epigenesis is transformed and carried forward into the twentieth and twenty-first centuries. In the first half of the twentieth century, theoretical concepts associated with epigenesis played a significant role in studies of development. This section surveys the main theoretical efforts along these lines.

Driesch’s experiments (see Section 7 ) demonstrated regulative development: the developing embryo compensated for experimental intervention rather than following a pre-determined plan. To account for this phenomenon without resorting to vitalism, a new theoretical approach was needed. One such was organicism, which Donna Haraway (1976) casts as a new Kuhnian paradigm for embryology in the first half of the twentieth century. Its tenets included:

• the organism is a unified whole, with multiple connections between levels of complexity/organization;
• development is a goal-directed process, regulated throughout rather than pre-determined; and
• organismal shape and structure should be understood in terms of principles for achieving form.

Lenoir (1982) traces these ideas to Kant’s Critique of Judgment , notably reciprocal determination of parts and the whole organism during the process of development. A more explicit influence was von Bertalanffy’s systems theory of development (1928 [1933, trans. Woodger]; see the entries on levels of organization in biology and systems and synthetic biology ). Another influence was D’Arcy Thompson’s seminal On Growth and Form (1917; see the entry on developmental biology ). A key idea was that organismal form emerges from a self-organizing system, not the unfolding of a pre-existing program.

Haraway (1976) identifies four main elements of organicism:

• the goal of explaining organismal form;
• concepts of symmetry, polarity, and pattern;
• field-particle duality (analogous to physics); and
• links to structuralism in philosophy (i.e., a structure as a self-regulating, complex system involving multiple interacting levels of organization and spatio-temporal scales).

She examines these elements in the work of the paradigm’s principal architects in twentieth-century developmental biology: Ross Harrison, Joseph Needham, and Paul Weiss. Organicists sought mathematical yet autonomously biological laws to account for emergent order in developmental processes. Although such laws did not materialize, Haraway argues that metaphors (principally those of crystals and fields) played significant theoretical roles in their stead. More abstract theoretical concepts—of polarity, gradients, and fields—were also used to explain organismal form in the first half of the twentieth century. For example, Hans Spemann (1936 [1938]) proposed that

the body of many, if not all animals, at least in the embryonic state, possesses one or several axes with unequal poles along which there exists a gradient of some sort. The course of development depends upon these gradients to a high degree. (1936 [1938: 318]).

Similarly, Charles Manning Child hypothesized that organismal development is based on “activity gradients” grounded in metabolism. Others objected that these concepts are unclear, metaphysically murky, and unscientifically vitalist (see Gilbert, Opitz, & Raff 1996).

An important spur to gradient and field theories was the phenomenon of “the organizer”, demonstrated by Hilde Mangold’s experiments transplanting newt tissue. She moved a small piece from the dorsal lip of a developing Triton cristatus embryo into the early gastrula of another newt species ( Triton taeniatus ). Donor tissue thus came in contact with undifferentiated ectoderm of another species. The result: two conjoined embryos with different body axes. This indicated that dorsal lip tissue somehow “organized” host cells and tissues into another embryonic body. Spemann designated the dorsal lip of the embryo “the organization-center” or “organizer”. But how did this work? The proposed answer suggested that body axes result from a gradient of some chemical substance emanating from cells of the organizer. This was the starting-point for “field theories”; e.g.,

…the egg and early embryo consist of fields—gradients or differentiation centers in which the specific properties drop off in intensity as the distance from the field center increases…. (Harrison 1969: 258)

In discussions with other members of Cambridge’s Theoretical Biology Club, Joseph Needham sought a theory of the chemical nature of the “organizer”. Building on D’Arcy Thompson’s theories of biological form and insights from biochemistry, he conceptualized fields as spatial regions of a developing embryo, which determine for all points within a region a specific quality, direction, and intensity. The field as a whole exhibits (in)stability and equilibrium positions. Organismal development is thus defined as

a progressive restriction of potencies by determination of the parts to pursue fixed fates…this state of affairs can best be pictured in the manner of a series of equilibrium states. (Needham 1936: 58)

The overall process is to be explained by probabilistic “field laws” describing a sequence of equilibrium positions for a field, extending from proteins and fibers to cell shape and organismal morphology. Although Needham did not flesh out his theoretical ideas in detail, they show clear parallels with Waddington’s view of development, as well as recent systems approaches (see Section 9.1 and section 10.2 ).

Working in the US, Paul Weiss also elaborated field and gradient concepts to sketch a theory of development that anticipated key ideas of systems biology today (see the entry on systems and synthetic biology ). His mature theory (“molecular ecology”) posited an array of molecular species in a cell, distributed, arranged, grouped in ways determined by their sterically-mediated interactions and physical environment (1968). Steric interactions are based on molecules’ shape and charge; biochemical features such as bonding and repulsion. Weiss theorized that these molecular interactions are organized by higher-level fields so as to produce patterns of cell differentiation, growth, and morphogenesis according to the principle of “gradual determination”. There is no pre-existing “seat” of embryonic organization in the egg or early embryo—instead, organization emerges in the process itself, through a physiological gradient. Weiss’s theory was based on extensive research on limb regeneration, neural development, grafting, and cell culture. A regenerated amphibian limb, for example, shows the correct orientation and organization of parts, even if derived from different cell types in another body part. How do the new parts acquire a “sense of direction” yielding correct morphological organization? Weiss’s answer is that some physiological gradient in the body, tells the cells “where they are and what to do”.

Weiss theorized this idea in terms of fields:

factors which cause the originally indefinite course of the individual parts of a germ to become definite and specific, and, furthermore, cause this to occur in compliance with a typical pattern. (1939: 290)

A field is organized from center to periphery, with a focal point of maximal intensity and gradual decrease from this center. This organization entails a “physiological gradient”: a robust distribution of (molecular) properties, which vary along the three spatial dimensions and along one or more axes (polarity). Its causal activity is holistic, determining cell behavior, which in turn produces growth, differentiation, and morphogenesis. Haraway (1976), Keller (2002), and Vecchi and Hernández (2014) examine the complex and uneven history of gradient and field concepts in developmental biology. Haraway, as discussed above, is concerned primarily with these concepts as metaphors structuring the organicist paradigm via the work of Harrison, Needham, and Weiss. Evelyn Fox Keller considers these concepts as part of the (so far largely unsuccessful) tradition of mathematicizing biology to understand development. Davide Vecchi and Isaac Hernández elaborate on Keller’s view, arguing that the concept of “morphogenetic field” was supplanted by Wolpert’s concept of “positional information”, part of the mid-twentieth-century theoretical shift from organicist ideas to genetic control of development (see Section 8.3 ).

Eschewing genetics, Turing’s theory of morphogenesis (1952) sought to explain how patterns first emerge in an early embryo. His answer was a simple model consisting of two reaction-diffusion equations, representing chemical “morphogens” X and Y diffusing through a tissue at rates determined by constants \(D_x\) and \(D_y,\) coupled to chemical reactions with effects represented by functions f and g :

(These equations follow Keller’s 2002 presentation, which is more streamlined than Turing’s original.) One morphogen is an activator, the other an inhibitor. Both are made and destroyed at constant rates. Initially, the tissue is homogeneous, lacking any spatial organization. Crucially, for stable patterns to emerge X and Y must diffuse at different rates, the inhibitor having the larger diffusion coefficient. Under these parameter values, random fluctuations in the system lead to instability resulting in pattern-formation. That is, from an initially stable, homogeneous system of cells/tissue, interaction between two diffusing chemicals (morphogens) leads to emergence of a stable chemical pattern. Turing assumes that steady-state chemical patterns determine patterns at the cell/tissue level (through mechanisms wholly unspecified). Given that assumption, the model accounts for pattern formation in a group of cells or a tissue.

Turing’s model of morphogenesis has several striking features. First, it is based on physics and chemistry rather than biology: reaction-diffusion equations incorporating Fourier series and the law of mass action. Reaction-diffusion equations are a class of non-linear partial differential equations. (Hodgkin and Huxley’s model of neural propagation is also of this class—at the time, one of the few applications of these equations to biology; see the entry on mechanisms in science .) Turing’s use of these equations to model the transition from homogeneity to spatial patterning is rather counterintuitive, as diffusion is a characteristically homogenizing process. So his model was a theoretical advance, extending the insight that reaction-diffusion equations can model transitions and “waves of advance” (e.g., in neurons) to morphogenesis (Berestycki 2013). Second, the model is strikingly abstract—“…a simplification and an idealization, and consequently a falsification” (Turing 1952: 37). Yet it has many biological applications. Turing (1952) explored mainly simple cases of chemical diffusion through a tissue of fixed size. But he also applied the model to chemical waves on spheres to represent gastrulation. That more complex case, with three interacting morphogens, can produce traveling waves and out-of-phase oscillations. Turing also applied his model to plant development, specifically phyllotaxis (arrangement of leaves on a stem; see S. B. Cooper & van Leeuwen 2013). Subsequent generalizations of Turing’s equations have been used to model a wide variety of developmental phenomena, including fish pigmentation patterns, human mesenchymal cell differentiation, butterfly wing patterns, mouse hair follicles/coat color patterns, limb and tooth development, and bacterial colony growth dynamics (Meinhardt 1982, Murray 1989). However, its explanatory role is controversial, primarily because it is hard to show that Turing’s model describes how development actually proceeds in real organisms.

Keller (2002) discusses this issue, as well as major features of Turing’s model. One key problem (noted by Waddington at the time) is that “pure homogeneity” as a starting point for development is unrealistic; the egg or early embryo is never a “blank slate” as Turing assumed. She argues that Turing’s was a premature attempt at mathematical biology, noting as well the challenge it posed to prevalent ideas about causality: organismal form as emergent, self-organizing; holistic, arising from the system rather than driven by genes. Vecchi and Hernández (2014) build on Keller’s view, diagnosing “a clash of causal ideologies” in the different possible interpretations of initiating cause in Turing’s model. They are more optimistic about its biological relevance and influence. Scientists continue to apply and reflect on Turing’s model of morphogenesis (e.g., Kondo & Miura 2010). As the latter scholars note, Francis Crick’s source-sink model of embryonic development (1970) can be considered the simplest form of a reaction-diffusion model, with the reaction term removed. However, Crick’s model is mechanistically simpler, with morphogenesis controlled by a single morphogen diffusing along a line of cells:

At one end of a line of cells one postulates a source—a cell that produces a chemical (which I shall call a morphogen) and maintains it at a constant level. At the other end the extreme cell acts as a sink: that is, it destroys the molecule, holding the concentration at that point to a fixed low level. The morphogen can diffuse from one cell to another along a line of cells. (Crick 1970: 420)

Unlike Turing’s more holistic model, in which patterns emerge from initial random instability due to interaction of multiple morphogens, Crick’s model grounds morphogenesis in simple chemical diffusion. Agutter, Malone, and Wheatley (2000) and Vecchi and Hernández (2014) argue that Crick’s model lacks explanatory power. However, Minelli (2009) suggests an updated source-sink model, informed by advances in molecular and cell biology.

More influential for mainstream developmental biology was the concept of positional information. Though originally a contribution to regulative theories of development, the concept soon transformed to support theories of genetic control. This epitomizes the shift in biological thought that followed breakthroughs on DNA structure, protein coding, and other phenomena of molecular genetics (see the entry on molecular genetics ). Lewis Wolpert (1969) introduced the concept of positional information to solve (like Turing)

the problem of assigning specific states to an ensemble of identical cells, whose initial states are relatively similar, such that the resulting ensemble of states forms a well-defined spatial pattern. (1969: 4)

Wolpert’s answer was that each cell in a developing system has a unique value reflecting its position in that system, encoded by the spatial gradient of a diffusible molecule across the system. Specification of a cell’s position in the system is relative to one or more points in the system: the source(s) of the diffusible molecule or cell property. Cells that share the same set of points relative to which their position is specified make up a field. Wolpert used this model to explain a range of experimental results and observations of development in various species (as well as the toy “French flag” example). In 1969, Wolpert saw the next task as discovering and articulating rules (and underlying mechanisms) for specification of positional information and polarity, the nature of points and boundaries, and “new meaning to classical concepts such as induction, dominance and field” (1969: 1).

Wolpert’s theoretical approach differed from predecessors in its relation to genetic information. Positional information was proposed as a “universal mechanism” for “translation of genetic information into spatial patterns of differentiation” (Wolpert 1969: 1). The concept was designed to integrate gene-based and organicist approaches to development. Cells “interpret” changes in their positional information, so as to change the pattern of gene activation in a cell. Changing patterns of gene expression drive cell differentiation in a developing organism. This in turn effects spatial distribution of cell forces (e.g., contraction, motility, cell-cell contact), which drives organism-level morphogenesis. However, by 1975, Wolpert’s thinking about development had undergone a conceptual shift (Keller 2002). Between 1971 and 1975, Wolpert’s ideas were “geneticized”, after which he accepted the idea of a genetic program for development and the embryo as “computable” from an egg. In this way, the idea of a morphogen gradient establishing positional information for cells was put under genetic control (see Section 9 ).

Wolpert and Lewis (1975) proposed a research program of computational embryology aimed at a

theory of development [that] would effectively enable one to compute the adult organism from the genetic information in the egg. The problem may be approached by viewing the egg as containing a program for development. (1975: 14)

Philosopher Alexander Rosenberg (1997) endorses this version of Wolpert’s theory as vindication of strong molecular reductionism in developmental biology. Manfred Laubichler and Günter Wagner (2001), alongside other biological theorists, reject this version of reductionism (see Section 10 ). Jaeger, Irons, and Monk (2008) propose to reverse Wolpert’s conceptual shift with their “relativistic theory of positional information”, adding a role for regulative feedback by responding cells. Vecchi and Hernández (2014) criticize Wolpert’s and Rosenberg’s reductionist computational embryology as classically preformationist and note explanatory lacunae of the idea of a genetic program pre-established in the egg. This follows Keller (2002), who argues that the twentieth-century consensus on development is essentially preformationist. This brings us to

the widespread reductionist and deterministic view of development as programmed in genes…with the implicit idea that the final form of the organism is “already there” in the instructions contained in its genome as early as the egg stage. (Minelli & Pradeu 2014b: 4)

9. Twentieth-Century Genetics: A New Predeterminism

Early twentieth-century embryology highlighted epigenesis. But a new twist on preformationism soon arose in the form of genetics. This pointed to the nuclear chromosomes as loci for the causes of differentiation. Yet unlike Weismann and Roux, new geneticists did not see the genetic material as divided up into a mosaic to explain ensuing cell and tissue differences. Rather, the inherited nuclear material was the same in every cell, but it acted differently according to an internal program. This interpretation appealed to some scientists for metaphysical reasons since it focused on the material units of heredity and apparently of causation. Epistemologically, it was more difficult to point to evidence that inherited genes explain development.

This is not the place for a history of genetics, which has been offered many times elsewhere. The important point here is that genetics brought a new form of preformationism. Instead of a dynamically acting organism taking its cues from the environmental conditions and from the way that cells interact with each cell division, the twentieth century brought a dominant and popular view that has often emphasized genes as programmed to carry the information of heredity, which was also the information necessary to construct an individual. Of course, there have been calls for alternatives or interactionist models where heredity and development, epigenesis and preformation, work together, but these have often been offered as alternatives than as central interpretations (see Section 10 ).

In the beginning of the twentieth century, at first Thomas Hunt Morgan resisted the Mendelian-chromosome theory of inheritance that saw inherited units of heredity carried on chromosomes as determinants of developed characters. If all the cells contain the same chromosomes, then how can their inheritance explain anything, he asked. Instead, he insisted that “We have two factors determining characters: heredity and the modification during development” (Morgan 1910a: 477). Morgan wrote to a friend that “my field is experimental embryology” and not the genetics with which he became associated (Morgan 1908). Like his Woods Hole colleagues at the Marine Biological Laboratory, Morgan did not see how inherited chromosomes could explain development of form from non-form. He had rejected Weismann’s interpretations and continued to reject the idea of inherited determinants.

Also in 1910, however, he was studying many different kinds of organisms in pursuit of explanations of heredity as well as differentiation. A white-eyed male Drosophila fly famously caught his attention, and led him to the conclusion that some inherited factors must, indeed, cause expression in the emerging organism (Morgan 1910b). It is not that Morgan changed his mind about how to do science, but rather that the evidence carried him in new directions (Maienschein 1991a).

The fertilized egg cell contains a nucleus made up of chromosomes inherited from both parents. Along these chromosomes are lined up units of heredity that serve like Weismann’s determinants, now called genes. These genes correlate with some characters in the resulting organism, and therefore in some sense the resulting form was predetermined in the egg. Yet it was not already formed. And, indeed, the mere correlation of genes and characters tells us virtually nothing about how the differentiation occurs nor about how form becomes formed (the problem of morphogenesis). Therefore, yes, genetics brings a sort of new preformation or more accurately predeterminism. But that in itself brings a description and a correlation but no explanation. Or so Morgan initially felt, as did his embryologist colleagues. Advances in genetics soon changed the theoretical landscape. In 1934, Morgan posited differential gene activity as the key to understanding organismal development. On his view, genes are inherited, their activity controlled by cytoplasmic factors. The latter are originally heterogeneously distributed in the egg. Differential gene activity so-driven leads in turn to differential cytoplasmic factor distribution, and the cycle continues, influenced by signals from a cell’s environment (primarily neighboring cells). Ideas of preformation and epigenesis are combined in pre-determined genes acting in response to diverse environmental factors. Burian (2005) argues that the old theoretical opposition was reinscribed in the disciplinary separation of embryology and genetics, particularly in the US. In that context, studies of development emphasizing the nucleus endorsed preformationism, while those focusing on cytoplasm and cell lineage differentiation tended to epigenesis. This theoretical rift was healed when research focus shifted from “cytoplasm vs. nucleus” to mechanisms of gene expression (Burian 2005). Theoretical concepts grounded in genetics and molecular biology thereafter increasingly dominated thinking about development.

By mid-twentieth century, and especially after the discovery of DNA’s structure (which apparently also explained its function), researchers began to forget or at least ignore questions about morphogenesis and epigenesis (see, for example, Olby 1974, Judson 1979). Instead they asked how, actually, genes give rise to differentiated form? Somehow that works, seemed to be the answer. Genetics predominated over what C.H. Waddington referred to as the epigenetics of development (Waddington 1942). Of course not everyone ignored development, but it became a seriously neglected field and even professional societies and journals that had focused on “embryology” shifted to “developmental genetics” (Oppenheimer 1967). When Robert Briggs and Thomas King cloned frogs in the 1950s and John Gurdon extended the research in the 1960s, it seemed that nuclear transfer could come from only early stages of development. Furthermore, the resulting clones were like their donors from whom the nuclei came rather than like the mothers from whom the eggs came (Briggs & King 1952; Gurdon & Colman 1999; McLaren 2000). Apparently development brings differentiation that is unidirectional. Preformationist/ predeterminist thinking prevailed. Epigenetic development and regulatory response to environmental conditions seemed to have strict limits for those adopting the mid-twentieth-century predeterminist emphasis.

In this context, Waddington’s theoretical approach was distinctive, aiming to integrate genetics and epigenetic development. His main device for doing so was “the epigenetic landscape”, first articulated in 1939 and iconically diagrammed in The Strategy of the Genes (1957). Waddington’s landscape model represents developmental potential as

a more or less flat, or rather undulating surface, which is tilted so that points representing later states are lower than those representing earlier ones … Then if something, such as a ball, were placed on the surface, it would run down toward some final end state at the bottom edge. (1957: 29).

The model visualizes three “essentially formal” properties of development: unidirectionality in time (via the landscape’s tilt), multiple discrete termini from a single undifferentiated start (via branching tracks), and robustness of developmental processes (via steepness of valley walls).

These topographic features reflect generalizations about animal development, based mainly on experiments on chick and Drosophila. A rolling ball’s path down the incline corresponds to the development of some cell or tissue from an early undifferentiated state to a mature differentiated state.

What controls the path taken by a cell or piece of developing tissue—a pre-determined genetic program or ongoing orchestration of stimuli? Waddington (1939, 1940, 1957) identified genes as the determinants of epigenetic landscape topography. This is depicted in a companion diagram showing the landscape’s “underside” as a network of interacting biochemical products “which are ultimately controlled by genes” (1957: 36). Waddington first articulated the landscape analogy in 1939, as a generalization of time- and dose-effect curves representing the role of genes in producing specific phenotypic effects. The landscape model results from including interactive effects of the entire genome on a particular pathway:

[o]ne might roughly say that all these genes correspond to the geological structure which moulds the form of the valley. (1939: 182)

Although he featured genetic control, Waddington’s designation of the landscape as “epigenetic” suggests a view of development allowing more of a role for epigenesis than more gene-focused contemporaries. Scott Gilbert (1991) argues that the branching-track lineage structure unifies development and genetics, visualizing a formal analogy between cellular, genetic, and organismal development. Fagan (2012, 2013a) extends this argument, focusing on Waddington’s unification of two complementary landscape images: robust pathways above, and interacting gene products below, with genes at the bottom, metaphorically “pulling the strings”. Nicoglou and Merlin (2017) and Nicoglou (2018) discuss this unifying, integrative feature of Waddington’s landscape model as one strand of twentieth-century epigenetics.

Waddington’s own later theorizing took a mathematical turn, interpreting the notion of a field in terms of spatial relations and using Réné Thom’s topology to theorize developmental pathways as an epigenetic system (1968). This work emphasized the concept of “chreod”, a trajectory (directed path) of normal development within multidimensional space, with axes for time, three spatial dimensions, and concentrations of chemicals. Although not fruitful in biology at the time, Waddington’s mathematical approach anticipated twenty-first-century alternatives to the idea of genetic control (see Section 10 ). In that context, the landscape model has experienced a renaissance. The underside image is replaced by complex gene regulatory networks, topside surfaces explicated in terms of systems biology and/or stem cell concepts (Fagan 2012, 2013a). Fusco and colleagues (2014) discuss recent theoretical proposals (e.g., Wang, Wang, & Huang 2010) to “rehabilitate” Waddington’s landscape as a dynamical systems model, which require significant changes from the original. They conclude that landscape models (updated or not) are good for visualizing cell differentiation but not other important developmental processes—and so not well-suited for a comprehensive theory of development. As a metaphorical aid to understanding, however, Waddington’s landscape remains influential. Jan Baedke (2013) examines its broader impact across the life sciences, extending beyond organismal development to developmental psychology and cultural anthropology.

In his influential book What is Life (1944)? Erwin Schrödinger speculated that development is controlled by “a code-script structure” localized to chromosomes, information-patterns

instrumental in bringing about the development they foreshadow…architect’s plan and builder’s craft all in one. (1944: 18–19)

Twentieth-century triumphs in genetics bolstered this idea. The main challenge for such theories is how invariant genes, the same in every cell of an organism, can control the process of diversification of those cells, their temporally-orchestrated movements and organization into tissues, bodily structures, and complex organs. Frank Lillie (1927) put the point starkly (discussed in Burian 2005):

The essential problem of development is precisely that differentiation in relation to space and time within the life-history of the individual which genetics appears implicitly to ignore… Those who desire to make genetics the basis of physiology of development will have to explain how an unchanging complex can direct the course of an ordered developmental stream. (1927: 365–367)

Responses to what Burian has labelled “Lillie’s paradox” hinge on differences in gene expression among cells and tissues. Chromosomal DNA is a linear template for an mRNA transcript, which is in turn a template for a sequence of amino acids making up a protein. Cell phenotype depends on which genes are transcribed and then translated. But DNA does not express itself. The challenge then is to account for gene expression in a way that preserves genetic control of development. The concept of a “genetic program for development” responds to this challenge.

The idea was forcefully articulated by François Jacob and Jacques Monod:

During embryonic development, the instructions contained in the chromosomes of the egg are gradually translated and executed… The whole plan of growth, the whole series of operations to be carried out, the order and the site of syntheses and their coordination are all written down in the nucleic-acid message. (Jacob 1970 [1973: 313])

Jacob and Monod’s view was grounded on their pioneering work on bacterial gene expression (1961a, 1961b). Their key result, the operon model, explains how gene expression in E. coli bacteria is regulated by environmental signals. Briefly, the model represents interactions among various components: protein-encoding DNA sequences, regulatory DNA sequences, the “operator region” of DNA located near the start of protein-encoding genes, repressor protein, and small molecules (e.g., allolactose). If a repressor binds the operator region, this blocks protein synthesis at nearby coding sequences. Small molecules like allolactose bind the repressor, inhibiting its inhibition, so the protein-encoding genes associated with the “de-repressed” operator are expressed. Those genes encode enzymes involved in metabolizing the small molecules, neatly closing the regulatory loop. Jacob and Monod singled out DNA as the controlling molecule of this regulatory system. Furthermore, as Michael Morange (2000) discusses, they extrapolated the E. coli operon model to gene regulation in general, positing that all organismal development is controlled by a small set of regulatory genes.

Jacob and Monod’s subsequent work emphasized molecular genetic reductionism and the idea of a genetic program for development. For example, Jacob began The Logic of Life (1970 [1973]) with reflections on “The Programme”:

… the structure of macromolecules is determined down to the last detail by sequences of four chemical radicals contained in the genetic heritage. What are transmitted from generation to generation are the “instructions” specifying the molecular structures: the architectural plans of the future organism. They are also the means of executing those plans and of coordinating the activities of the system. In the chromosomes received from its parents, each egg therefore contains its entire future: the stages of its development, the shape and the properties of the living being which will emerge. The organism thus becomes the realization of a programme prescribed by its heredity. (1970 [1973: 1–2])

However, extrapolation from bacterial gene regulation to development in multicellular organisms proved unfounded. Limitations of the operon model soon became evident. In eukaryotic, multicellular organisms, gene expression is far more complex. Fifty years of molecular biology have revealed a menagerie of mechanisms implicated in the complex cellular machinery of gene expression (see the entry on molecular biology ). And this is not to mention interactions among cells and tissues, about which classic molecular biology was silent. Brian Goodwin, a student of Waddington’s, charted an alternative theoretical path that skirted these limitations. In the 1960s, he developed equations to model genetic oscillation, building on Jacob and Monod’s operon model (Goodwin 1965). But his subsequent theorizing about development emphasized self-organization due to chemical gradients and mechanical strain in cytoplasm, presaging dynamic and physical theories that gained prominence around the turn of the millennium (Goodwin & Trainor 1985, see Section 10.2 and the entry on systems and synthetic biology ).

The next experimental milestone for theories of genetic control of development was Christiane Nüsslein-Volhard and collaborators’ painstaking characterization of mechanisms of body patterning and polarity in Drosophila (e.g., Driever & Nüsslein-Volhard 1988). They distinguished three regions along the early embryo’s anteroposterior body axis (anterior, posterior, and terminal), the basic patterns and structures of which are controlled by distinct sets of maternal genes. A few such genes have long-range “organizing” effects. Notably, Driever and Nüsslein-Volhard (1988) showed that bicoid protein acts as a morphogen, controlling formation of a fly’s anterior (including head and thorax) in accordance with its concentration gradient. Significantly, their experimental manipulations were DNA sequence mutations (known to effect anterior body plan development or bicoid copy number). The experiments thereby charted a causal pathway from DNA to mRNA to protein to cell differentiation to morphogenesis. This demonstration of changes in body-plan correlated with maternal genetic mutations was powerful vindication of the idea that genes control development. Moreover, central concepts of the organicist alternative (gradients and positional information) were brought under the rubric of that theory. Nüsslein-Volhard’s gene-based approach remains influential in developmental biology (see the entry on developmental biology ).

The above and other experimental successes underpin Eric Davidson’s theory of “the regulatory genome” (2006), which combines new experimental data (mainly from Drosophila and sea urchin) with systems and network concepts—in effect, updating Jacob and Monod’s notion of the genetic program. On this view, development is controlled by a DNA-encoded program made up of short sequences that specifically bind transcription factor proteins and thereby make a difference to gene expression. The first step of gene expression in higher organisms is typically the binding of one or more protein factors to a regulatory region of DNA located near the sequence to be transcribed (“read” into mRNA and thence to protein). Davidson terms these DNA sequences “cis-regulatory modules”. Each module’s effect is represented as a conditional rule of the form: “If protein X is present, then gene Y is expressed at level Z”. Each gene has a set of such modules associated with it, which collectively specify its expression pattern under various conditions. That expression pattern, in turn, determines a cell’s phenotype and developmental fate. Control of development is therefore attributed entirely to the DNA components of molecular complexes that make a difference to gene expression. Transcription factor proteins and other components are conceived as “inputs” to the information-processing modules, and effects on gene expression as “outputs”.

These basic “cis-regulatory” units are organized into systems of interacting modules. Because transcription factor proteins are products of gene expression, the regulatory modules that control their expression are sites of “primary core control” for development. These core control modules form an interconnected network, reflecting the influences of transcription factor proteins on one another’s expression. All organisms have a genetic regulatory network composed of DNA sequences distributed throughout its genome, which constitutes a stable underlying program for development. Collectively, DNA modules act as “a vast, delocalized computational device” that processes regulatory states of a cell (Davidson 2006: 185). Furthermore, regulatory DNA sequences are (for the most part) invariant across cells of an organism, and organisms of a species, with a relatively small set of “hub” transcription factors conserved throughout much of the animal kingdom. The regulatory genome therefore offers a unified explanation of animal evolution and development. However, there are good reasons to think the role of DNA sequences in gene expression is not distinctive, causally or informationally (see the entries on gene , genetics , and reductionism in biology ). Other recent theoretical approaches tend toward more inclusive, less gene-centric explanations of development.

10. Twenty-First-Century Alternatives: A New Epigenesis?

The end of the twentieth century brought discoveries that challenged prevailing genetic determinism, and also began to replace the extreme forms of either preformationism or epigenesis with the sorts of interactionist models that were only offered as outlying alternatives in earlier decades. A modified form of epigenesis, in which the organism is seen as beginning from an inherited egg and sperm that do include genes, seems to be on the rise again. Research on cell development, stem cells, and tissue engineering shows that the identity of any particular cell is not predetermined, but also depends on interactions with its neighbors and context. Ian Wilmut’s team’s success in cloning Dolly, reported in 1997, and John Gearhart and James Thomson’s successes with developing human stem cell lines, both reported in 1998, challenged prevailing assumptions (Wilmut et al 1997, Wilmut, Campbell, & Tudge 2000; Thomson et al. 1998; Shamblott et al. 1998; Gearhart 1998). Both suggested that development is a good deal more flexible, plastic, and interactive than preformationist interpretations allow. These experimental results, alongside conceptual issues, spur alternative theories of development. Gilbert (2004) notes increased emphasis on interaction, change, emergence, and reciprocal “determination” relations between whole and component parts. His own work emphasizes the need for multiple perspectives in understanding development; notably diversification in the location of causal control beyond genes alone (see the entry on evolution and development ). This section focuses on philosophical efforts along these lines.

Susan Oyama’s monograph The Ontogeny of Information (1985 [2000a]) critiques gene-based theories of development. She argues that genetic information for development is causally derivative, existing as such only in the context of an ongoing developmental process. That process, in all its contingency and historical specificity, is causally primary. Neither DNA nor environmental factors are causes of development outside the interactive context that makes features of the physical world an organism’s environment and induces patterns of gene activation within cells. Genetic information emerges from the interactions of heterogeneous, dispersed, developmental resources. Central to Oyama’s view is the

conception of a developmental system, not as the reading off of a preexisting code, but as a complex of interacting influences, some inside the organism’s skin, some external to it, and including its ecological niche in all its spatial and temporal aspects. (1985 [2000a: 39])

As in earlier organicism, patterns and form emerge from ongoing interaction, within and among multiple levels of organization, on multiple time-scales. But unlike the organicists, Oyama has an established theoretical alternative to contend with. Much of her argument is negative, rejecting the gene/environment dichotomy, primacy of genes as causes, and lingering preformationism of the idea of a genetic program for development. Her theoretical perspective aims to overcome entrenched (and often unrecognized) dichotomies in biological thought. Although criticized for their complexity, apparent incompatibility with scientific generalization, and lack of connection to experimental practices, Oyama’s ideas about developmental systems have been influential in twenty-first-century philosophy of biology.

One impact is further articulation of developmental systems theory (DST; see the entries on inheritance systems and replication and reproduction ). In the introduction to their 2001 edited volume, Cycles of Contingency , Oyama, Paul Griffiths, and Russell Gray set out DST’s major themes and commitments: joint determination by multiple causes (causal parity), context sensitivity and contingency, extended inheritance, development as construction, distributed control, and evolution as construction. Another core tenet is rejection of opposed dualities such as nature/nurture, gene/environment, and biology/culture—not arguing that an adequate theory of development must include both sides of opposition, but that the oppositions themselves have no place in an adequate theory of development. Relatedly, genes are not privileged causes of development (Griffiths & Knight 1998). This “parity thesis” is sometimes misunderstood as a theoretical claim that all causes of development are equally important. But it is better understood as a constraint on theories of development (and evolution): to not presuppose any fundamental distinction between genes and all other causes (Oyama et al. 2001: 3). DST’s other themes are similarly arrayed against twentieth-century predeterminism. In this way, DST entails a conceptual reorientation in thinking about inheritance, development, and evolution, and a point of departure for philosophical exploration of alternatives to gene-centrism and entrenched dichotomies. Much of this research focuses on integrating development and evolution—i.e., evo-devo theories (see the entry on evolution and development ). DST-affiliated projects that focus on organismal development are discussed here.

A developmental system is

a heterogeneous and causally complex mix of interacting entities and influences that produces the life cycle of an organism. (Oyama 2000b: 1)

James Griesemer (2000a, 2000b, 2014a, 2014b) further articulates this idea by conceptualizing development as part of a more general theory of reproduction (see the entry on replication and reproduction ). In this theory,

development is the acquisition of the capacity to reproduce, where reproduction involves material propagation of developmental capacities from parents to offspring. (2014b: 199)

If development is so conceived,

…then it turns out all traditional life cycles are complex…[i.e.] there is typically at least one substantial change of developmental context or niche, with multiple generations of progenerants, before a life cycle is completed. (2014b: 191–192)

Complex life cycles of parasites and viruses, then, are not idiosyncratic but rather exemplars of complex, context-dependent developmental processes. Griesemer’s theoretical reorientation departs from the traditional idea of development as proceeding linearly from egg to embryo to adult. Instead, developmental capacities are properties of life cycles involving multiple organismal forms. This motivates a new theoretical concept: scaffolding interactions that bring a developing entity and aspects of the environment into such close association as to qualify as a new “hybrid” entity with new developmental potentialities (Griesemer 2014a). The original entity and environment are as “parents” to the new “hybrid” developmental system (e.g., HIV integrated into host genome vs. free virus and uninfected human cell/genome). Genes are demoted to “a special kind of evolved class of scaffolding developmental mechanisms” (2014b: 198).

Biologist Alessandro Minelli (2009, 2014) endorses a similar view of development, arguing that the first step toward a satisfactory theory is characterizing the full range of developmental processes, beyond the traditional “adultocentric perspective” tracing the formation of a mature organism from an egg cell. Although Minelli does not theorize development in terms of reproducers, he does conceptualize the process in terms of life cycles (see also Bonner 1974). Life cycles can be multigenerational, multigenomic, unicellular—and so developmental processes include these too, implicating haploid stages in sexually-reproducing organisms, alternation of generations, mixed sexual/asexual reproduction as in colonial tunicates and plants, etc. Minelli proposes to use this expanded view of “developmental disparity”, encompassing the full range of diversity of life cycles of living things, alongside “traditional, or naïve, concepts of development” to arrive at a satisfactory general framework (2014: 228).

Jason Scott Robert (2004) extends Oyama’s critique of the prevailing view that

development is now standardly construed as the epigenesis of something preformed in the DNA. (2004: 35)

This combination of old theoretical opposites, he argues, is unbalanced in favor of preformationism. Robert proposes instead “creative development”:

the organism’s semi-autonomous self-constitution from a range of ontogenetic raw materials,

co-constructed with its environment (2004: 87). Burian (2005) similarly challenges the adequacy of genetic determinism for understanding development, arguing that experimental results better support multilevel causation. Although grounded in experimental practices and associated concepts rather than abstract theory, Burian’s arguments motivate search for new theories to account for robustness of developmental processes and outcomes.

Another strand of theorizing development draws on physics of materials. Stuart Newman (2003) argues that physical factors played a major role in the early evolutionary history of multicellular organisms. Living tissues undergoing development have properties of liquid-like “soft matter”, such as viscous flow, elasticity, surface tension. They are also “excitable”,

simultaneously hav[ing] chemical, mechanical, and electrical properties, they behave in a multiscale fashion, continuously and simultaneously changing their composition and organizational properties on several temporal and spatial scales. (2003: 96)

These material properties allow living tissues to be analyzed in terms of physical theories and constraints. Although developmental processes in organisms today appear to follow pre-determined genetic programs, physical forces and constraints are at the origin of developmental systems and still play a vital role within them.

Newman’s theoretical approach “link[s] the modern physics of materials with contemporary knowledge of genetic determinants” (2014: 107; see also Newman 2003; Forgacs & Newman 2005). A core thesis of this theoretical approach is that excitable soft matter can exhibit self-organization in the absence of any pre-existing program; e.g., synchronized oscillations, local diffusion gradients, compartment-formation via cell adhesion. (This theoretical approach is anticipated in the work of Brian Goodwin, see Section 9.2 .) Newman posits that the earliest multicellular organisms underwent development spurred by these physical processes, together yielding a basic body plan. Over long evolutionary timescales, gene regulatory networks are overlaid upon physics-driven self-organizing processes. The result is a highly-conserved set of dynamical patterning modules (DPMs) that make basic shapes in a developing embryo: layers, lumens, folds, tubes, segments, etc. Importantly, DPMs are not predetermined genetic programs, but more inclusive systems capable of self-organization. Newman and colleagues use computer simulation to study the possible patterns generated by dynamical gene network models representing DPMs (Salazar-Ciudad, Newman, & Solé 2001). This method brings in dynamical systems theory, which has recently become central to systems biology (see the entry on systems and synthetic biology ).

Dynamical systems approaches to development, being committed to holism and temporal unfolding, echo classic epigenesis and early twentieth-century organicism. These earlier ideas are brought to bear on new high-throughput quantitative datasets using simulation technology—updating and revitalizing the earlier systems theoretic approaches of D’Arcy Thompson and von Bertalanffy. Although most work in systems biology focuses on single-cell organisms and intracellular networks, some researchers look to dynamical systems theory to construct a general theory of development. Following Goodwin’s theories, Johannes Jaeger and colleagues seek to identify a finite core set of basic developmental processes implemented by genetic-epigenetic-cellular regulatory systems (e.g., Jaeger & Sharpe 2014). They use computer simulation of regulatory network models to identify classes of geometrically similar networks that perform specific functions (such as producing stripe patterns). The goal is a rational classification scheme for functionally-characterized developmental mechanisms, which can in turn serve as groundwork for a general theory of development. It remains unclear, however, if biological functions in development can be characterized in a context-independent (modular) way, allowing for a tractable theory. Relatedly, Jonathan Bard (2011, 2013) posits a relatively small set of developmental networks driving “patterning, signaling, proliferation, differentiation, and morphogenesis”, “themes used over and over again” in networks of signaling pathways. Bard’s graphical network approach is distinctive in cutting across spatial scales and levels of hierarchical organization. Core processes (as networks) are distributed across levels; there is no preferred level of developmental activity, but all involved simultaneously. The resulting network structures are quite complex.

Results of stem cell experiments prompt re-examination of now-standard assumptions about the nature of development. At early stages of mammalian development when embryonic stem cells are most plentiful, in the blastocyst stage just before implantation, stem cells can be harvested and cultured to become a large number of different kinds of cells (Thomson et al. 1998). In theory, they have the capacity to become any kind of differentiated cell, but it is impossible to prove that conclusively (Fagan 2013b). Position in the organism and with respect to other cells seems to be decisive in directing differentiation. Although genetics provides information about the range of possibilities, regulation of gene expression involves diverse factors—and stem cells’ plasticity and context-dependence fits a more epigenetic view. Cell reprogramming, for example, is a method of producing stem cells (induced pluripotent stem cells) from more differentiated cells, effectively reversing normal development (Takahashi & Yamanaka 2006). This contradicts any strict principle of development as irreversible and internally-directed, undermining the notion of a fixed program for development (Brandt 2010). Melinda Cooper (2003) examines how central concepts of stem cell research (potency, differentiation, form, and regeneration) fit within the long tradition of theorizing about epigenesis and self-organization, dating from seventeenth-century revivals of Aristotelian embryology.

Ariane Dröscher (2014) traces the history of conceptual change concerning stem cells via lineage tree diagrams, a form of visual representation common to studies of development and of evolution. Lineage diagrams track relations between generations of reproducing entities. From Haeckel’s speculative image of the Tree of Life and Weismann’s introduction of “cytogenetic tree diagrams”, Dröscher examines this cell-centric way of understanding development from the mid-nineteenth century onward. Scott Gilbert (1991) shows that branching tracks have been used to represent cell development since the late nineteenth century, most prominently in cell lineage diagrams that track cell pedigrees and division events. In these models, branch-points represent cell division events and branching tracks represent genealogical relations among cells. Other cell characteristics are also represented, such as position within the developing embryo, morphology, and developmental fate. Waddington’s landscape is a model of this type (see Section 8.1 ). Fagan (2013a) uses this modeling approach to characterize cell development in general. Any process of cell development produces a cell lineage tree with a specific topology, representing developmental and reproductive relations between cells. Different stem cell concepts correspond to models that differentially constrain the space of possible topologies for cell lineage trees. This framework provides a systematic representation of the many different varieties of stem cell.

Fagan (2013a, 2017) extends this cell lineage framework to cell-molecular and cell-organism relations, yielding a cell-centric multi-level model of development. Her view of the cell-molecular relation is that of systems biology: a cell state is a pattern of gene expression and molecular interactions that determines a cell’s structural and functional characteristics. The cell-organism relation is “enkaptic”—that is, cells reproduce and differentiate (the two defining stem cell abilities) and are collected or encapsulated into a higher-level system: gastrula, tissue, organ, or an experimentally-produced organoid or embryo-like structure. The higher-level system emerges via formation of a boundary separating it from its environment. So different levels or scales of a developmental process are related by “boundary-formation”. On this view, development is a process comprised of multi-layered interactive networks, arranged in strata (layers) related by an encapsulating boundary. Each network is related to the one below by boundary-formation, conceived as emerging out of (and imposing constraints on) smaller-scale interactions such that an encapsulating system is distinguished from its environment.

Jane Maienschien and Kate MacCord offer a suggestive look at regeneration as a response of complex systems to injury through repair. (Maienschein and MacCord, 2022) The repair draws on inherited and in some ways predelineated organization, as well as on responses to changing environments. Both Fagan’s stem cell-based alternatives to predetermined development and Maienschein and MacCord’s suggestions about regeneration across all scales of life hark back to Morgan’s suggestion that

a process of pure epigenetic development, as generally understood nowadays, may also be predetermined in the egg. (Morgan 1901: 968)

The nowadays of the twenty-first century may take us back to some of the understanding and insights of the early twentieth, a time when a balance of epigenesis and preformation seemed likely.

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The authors would like to thank Alan Love and Karin Ekholm for helpful and constructive comments on a earlier draft. This entry includes portions of the former entry on Epigenesis and Preformationism (Spring 2017) .

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23: Human Growth and Development

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• Suzanne Wakim & Mandeep Grewal
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This chapter describes how the human organism grows and develops from fertilization through death. The following stages of life are described in detail: germinal stage, embryonic stage, fetal stage, infancy, childhood, adolescence, and adulthood.

• 23.1: Case Study: How Our Bodies Change Throughout Life Paul and Vanessa are shocked to discover that their toddler Lucas' blood lead level is 10 µg/dL, which is considered high. Since Vanessa is three months pregnant, they are worried about whether Vanessa was also exposed to lead. If so, what effects could it have on the developing baby?
• 23.2: Germinal Stage The germinal stage of development is the first and shortest of the stages of the human lifespan. The main events in this stage of development are illustrated in the figure below and described in detail in the rest of this concept. The germinal stage lasts a total of eight to nine days. It begins in a Fallopian tube when an ovum is fertilized by a sperm to form a zygote (day 0). The germinal stage continues as the zygote undergoes several initial cell divisions to a morula.
• 23.3: Embryonic Stage In many cultures, marriage - along with birth and death - is considered the most pivotal life event. For pioneering developmental biologist Lewis Wolpert, however, these life events are overrated. According to Wolpert, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Gastrulation is a major biological event that occurs early in the embryonic stage of human development.
• 23.4: Fetal Stage This mother-to-be is holding an ultrasound image of her fetus. She is nearly nine months pregnant, so the fetus is fully developed and almost ready to be born. The fetus has grown tremendously and changed in many other ways since it was a tiny embryo seven months previously.
• 23.5: Infancy Infancy refers to the first year of life after birth, and an infant is defined as a human being between birth and the first birthday. The term baby is usually considered synonymous with infant, although it is commonly applied to the young of other animals, as well as humans. Human infants seem weak and helpless at birth, but they are actually born with a surprising range of abilities. Most of their senses are quite well developed, and they can also communicate their needs by crying.
• 23.6: Childhood Legally, childhood is defined as the period of minority, which lasts from birth until adulthood (majority). The age of majority varies by place and purpose. For example, in the United States, at age 18, you are considered an adult for military service, but a minor for buying alcohol. Biologically, childhood is defined as the stage of a human organism between birth and adolescence.
• 23.7: Adolescence and Puberty Adolescence is the period of transition between childhood and adulthood. It is generally considered to start with puberty, during which sexual maturation occurs and adolescents go through a spurt in growth. In many children, however, puberty actually begins during the stage called pre-adolescence, which covers the ages 11 to 12 years. Puberty may begin before adolescence, but it usually continues for several years, well into the adolescent stage, which ends during the late teens.
• 23.8: Adulthood This family image includes an elderly woman and her young-adult daughters and granddaughters from the Hmong ethnic group in Laos. Grandmother and daughters are adults, but they are obviously far apart in age. What ages define the beginning and end of adulthood?
• 23.9: Case Study Conclusion: Lead Danger and Chapter Summary Earlier in this chapter, you met Paul, Vanessa, and Lucas, who were concerned by elevated levels of lead in Lucas' blood. Many experts agree that preventing lead exposure and more widespread blood lead level screening is critical to prevent permanent damage to children’s health. Infancy and early childhood is a wonderful time of tremendous growth and change in a person’s lifespan, but it is also a time that is highly vulnerable to damage—with potential lifelong consequences.

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• Published: 29 July 2021

The evolution of our understanding of human development over the last 10 years

• Ali H. Brivanlou   ORCID: orcid.org/0000-0002-1761-280X 1 &
• Norbert Gleicher   ORCID: orcid.org/0000-0002-0202-4167 2 , 3 , 4  

Nature Communications volume  12 , Article number:  4615 ( 2021 ) Cite this article

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• Developmental biology
• Embryogenesis

As it fulfills an irresistible need to understand our own origins, research on human development occupies a unique niche in scientific and medical research. In this Comment, we explore the progress in our understanding of human development over the past 10 years. The focus is on basic research, clinical applications, and ethical considerations.

What basic research has taught us about human development

Over the last decade, progress in understanding our own development was mostly driven by the emergence and combination of remarkable new technologies. New molecular biology tools such as single-cell RNA-sequencing (sc-RNA-seq) unveiled the earliest genetic signature of the three cell lineages of the human blastocyst and allowed for the discovery of human-specific signatures 1 , 2 , 3 . CRISPR/Cas9 genome editing has offered further access to in vitro functional studies in human blastocysts 4 . However, as we discuss below, an ethical line was crossed when a group claimed that genetically modified human embryos had been transferred, leading to births 5 when neither public opinion nor a consensus within the scientific community had been reached regarding whether crossing the germline in in vitro fertilization (IVF) was safe and ethically acceptable.

On the embryology side, the development of an in vitro attachment platform for human blastocysts offered a first glance into post-implantation events up to 12 days 1 , 3 , 5 , 6 . This paved the way for several important discoveries, including the observation that the human embryo can self-organize to generate embryonic and extraembryonic germ layers, yolk sac, and amniotic cavities in the absence of maternal influences 5 , 6 ; and the presence of a transient embryonic tissue of trophectodermal lineage, adjacent to the yolk sac, therefore named, yolk-sac trophectoderm ( ysTE ) 5 . The presence of these seemingly human-specific populations was independently confirmed by sc-RNA-seq 1 .

The marriage of stem cell biology with bioengineering gave birth to the field of synthetic embryology 7 , 8 , 9 , 10 , 11 , 12 , 13 . This technology uses human embryonic stem cells (hESCs) cultured on geometrically confined micropatterned substrates to generate 2D in vitro models of human conceptuses, such as models of the gastrula ( gastruloids ) 7 , 8 , 9 , 10 , 11 , 12 , 13 , or parts of the embryo, such as cerebroids and neuruloids 14 . Thousands of nearly identical self-organizing human embryonic structures allow for standardization and reproducibility, which cannot be achieved in standard organoid structures 15 . Cells within these structures can be tracked and quantified in real time with sub-cellular resolution, using sophisticated quantification code, including artificial intelligence 14 .

Human gastruloids induce formation of the primitive streak and have enabled the deciphering of the molecular network underlying gastrulation—the most crucial moment of our lives 7 , 8 , 9 , 10 , 11 , 12 , 13 . 3D models of human epiblasts can spontaneously break axial symmetry, thus providing an assay for the elucidation of molecular events underlying the emergence of antero–posterior polarity 11 , 16 . A highly homogenous population of self-organizing 3D models of amniotic ectoderm-like cells can be obtained by combining microfluidic and microculture approaches 17 .

Finally, the development of interspecies chimeras provided the most stringent in vivo validation of human embryo models 9 , 10 , 18 . Unimaginable in human models, inter-species chimeras have become the next best choice to test whether hESC behavior in self-organizing gastruloids , as observed on microchips, would also occur in an embryonic environment 10 , 18 , 19 . Human/bird chimeras generated from transplanting human gastruloids into early chick embryos in ovo unexpectedly proved more efficient than previous methods 9 , 19 . They allowed for the observation of an entire self-organizing embryonic axis in bird eggs 9 . As birds are closer to dinosaurs than to humans, this high rate of success with these chimeras further suggested that these early patterning events must be highly conserved.

Translational clinical applications that arose from basic research

The past 10 years bore witness to significant clinical progress in reproductive medicine, often translated from basic research. Successful human uterus transplantation and the subsequent birth of healthy offspring was, for example, only achieved after years of meticulous laboratory work in animals 10 . Significant improvements in cryopreservation technology for human eggs and ovarian tissue were also preceded by research in model systems 10 , 20 . Practical clinical applications have been developed for women in need of cancer treatment that are toxic to ovaries. In these cases, oocytes and/or ovarian tissue can be cryopreserved for later use in fertility treatments once the patient is cured of her cancer 21 . This ever-evolving technology has already proven to result in live births, and has also become an integral part of routine infertility treatments with IVF, giving rise to the brand-new concept of fertility extension through egg-freezing.

Diagnostic technologies to assess retrieved eggs and preimplantation-stage embryos in the IVF process have been disappointing. For example, tracking extended embryo culture to blastocyst-stage with time-lapse imaging failed to improve embryo selection 22 . That chromosomal-abnormal embryos increase with maternal (but not paternal) age has been interpreted to mean that chromosomal abnormalities were a principal cause for lower implantation chances and higher miscarriage risks among older women. This assumption led to the rapidly growing utilization of chromosomal testing of human embryos prior to embryo transfer in a procedure recently renamed preimplantation genetic testing for aneuploidy (PGT-A) 23 . The hypothesis behind PGT-A is to exclude chromosomal-abnormal embryos from the transfer, thereby improving implantation potentials of remaining euploid embryos.

Here too, clinical evidence was unable to confirm the hypothesis 24 . Moreover, basic research demonstrated a self-correction mechanism in mouse 25 and human embryos 26 , 27 , 28 , 29 that arose during embryogenesis that was cell lineage-specific to the embryonic cell lineage. In contrast, PGT-A biopsies are obtained from the extraembryonic-derived trophectoderm, rendering any diagnostic procedure at the blastocyst stage ineffective. In addition, mathematical modeling demonstrated that results from a single trophectoderm biopsy could not be extrapolated to the whole embryo 30 . Transfer of PGT-A “chromosomal-abnormal diagnosed embryos” has resulted in the births of over 400 chromosomal-normal offspring 20 , 21 .

In recent years, increasing attention has also been given to the quickly evolving understanding of how interdependent lifestyle and human fertility are 31 , 32 , 33 , including the influence of diet on the microbiome, as in many other areas of medicine.

The ethical significance of understanding human development

Whether in clinical medicine or in the research laboratory, human embryology has remained an ethical minefield, strongly influenced by socio-political and religious considerations. At the core of the controversy resides the special moral value of the human embryo, a subject that has come to the forefront again with the ascent of human embryonic stem cell research 34 . There is, however, little consensus as to how to answer a previously raised question: “ what is an embryo ?” 35 . The term pre-embryo, first introduced in 1986, was defined as the interval up to the appearance of the primitive streak, which marks biological individuation at ~14 days post-fertilization. This definition designated the period beyond 14 days as the time when a pre-embryo attains special moral status 36 , 37 . Paradoxically, the term pre-embryo has been replaced by the indiscriminate use of the term embryo, whether at preimplantation cleavage or blastocyst-stages or post-implantation before day 14. It was suggested that the distinction was important for ethical, moral, and biological relevance. The principal reason is simple: Until a pre-embryo becomes an embryo, there is no way of knowing whether implantation has taken place, whether a pregnancy is developing, whether there is a single pregnancy or twinning, or whether fertilization ended up in a benign (hydatidiform mole) or even in a malignant tumor (choriocarcinoma) 35 . Assigning advanced moral value to embryos at those early stages is, therefore, difficult to defend.

The past 10 years have witnessed innumerous ethical debates related to this subject, each with its own social, historical, and religious justifications, reflecting cultural diversities in human populations. Most are triggered by scientific breakthroughs. We summarize here the major ethical challenges preoccupying reproductive research and clinical practice.

We have already briefly referred to CRISPR/Cas9 genome editing. While the use of sc-RNA-seq to identify the molecular blueprint of human development has not elicited significant controversy, CRISPR/Cas9 genome editing of human embryos has been a topic of intense discussions and is currently permissible only in vitro 38 . An alleged attempt in China of implanting human genome-edited embryos into the uterus supposedly led to two births (one a twin birth). Though widely discussed in the media, neither attempt was published in the medical literature, and therefore cannot be verified 5 , 38 .

The ethical debates surrounding the 14-day rule, quiescent since the early IVF days, experienced a rebirth that was prompted by in vitro human embryo attachment studies and the emergence of synthetic human embryos. Within this context, we note that self-organizing embryo models are nothing more than cells in culture and are certainly not embryos. Regardless of scientific merits, in the U.S., the National Institutes of Health (NIH) currently prohibits the use of public funds for the study of synthetic embryos “for ethical reasons”. After being under an NIH moratorium for more than a year, research on chimeras is now, however, again permitted, though human/non-human primate chimeras remain prohibited.

These ongoing ethical debates mostly also mirror those surrounding the lack of U.S. federal funding for clinical IVF and related research, as well as hESCs-derived model embryos. In this context, the American Society for Reproductive Medicine (ASRM)’s Ethics in Embryo Research Task Force recently made an important statement: “ Scientific research using human embryos advances human health and provides vital insights into reproduction and disease ” 39 .

Provided certain guidelines and safeguards are followed, research with already existing embryos or embryos specifically produced for research should be ethically acceptable as a means of obtaining new knowledge that may benefit human health. ASRM also pointed out that scientists and society must understand which research questions necessitate the use of human embryos.

It is gratifying to acknowledge the history and vitality of ongoing debates, especially since they increasingly mimic decision-making processes in the medical field. These debates are meant to be based on cost-benefit and/or risk-benefit assessments. These debates will, unquestionably, continue and, indeed, considering that every intervention has consequences, must be decided based on careful considerations, including all relevant stakeholders and all parts of society.

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Acknowledgements

We like to thank Min Yang, Jean Marx Santel, Adam Souza, and Amir Brivanlou, for data gathering and critical reading of the manuscript, and constructive criticism.

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Brivanlou, A.H., Gleicher, N. The evolution of our understanding of human development over the last 10 years. Nat Commun 12 , 4615 (2021). https://doi.org/10.1038/s41467-021-24793-3

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4 Biological Development: Theoretical Approaches, Techniques, and Key Findings

Robert Lickliter, Department of Psychology, Florida International University, Miami, FL

• Published: 16 December 2013
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The relation between genotype and phenotype was thought to be relatively straightforward for most of the last century. The majority of biologists assumed that the instructions for building organisms were present in their genes and that genes were also the exclusive means by which these instructions were transmitted from one generation to the next. As a result of these assumptions, few biologists believed that development had any relevance to evolution. In this chapter I explore what brought about this genocentric view of development and evolution and review how recent advances across the life sciences challenge these longstanding assumptions. Genes are certainly critical to all development, but it is increasingly clear that the passing on of genes cannot fully account for phenotypic outcomes or for evolutionary change. Genetic and nongenetic factors constitute a dynamic developmental system, and evidence from contemporary epigenetic research indicates that it is not biologically meaningful to discuss gene activity without reference to the molecular, cellular, organismal, and environmental context within which genes are activated and expressed. This key insight suggests the possibility of bringing together molecular, developmental, and evolutionary biology in one dynamic model of the phenotype. The implications of this effort for developmental psychology are explored.

Comprehensive theories of biological development must account for each of its fundamental features, namely the emergence of complexity of organization by differentiation, the stability of form and function across generations, and the origin and range of variability across individuals.

For much of the twentieth century, development was characterized as the process by which genotypic specification is translated into the phenotypic traits of individuals, including their anatomy, physiology, and behavior. The environment was thought to play only a minor role in development.

The assumption that genetic factors ultimately determine developmental outcome provided the overarching causal structure used to articulate ideas about heredity and evolution for most of the last century. This assumption has been challenged and rejected by recent advances in the biological sciences.

While it is certainly the case that gene activity is involved in the production of all phenotypic traits, studies from the rapidly growing fields of epigenetics and evolutionary developmental biology consistently show that the environment plays a fundamental role in development for all species.

Gene expression is now understood as a dynamic and contingent process that regularly involves factors external to DNA. These include multiple factors within the environment of the cell as well as multiple factors external to the cell, many occurring beyond the boundary of the organism. Epigenetic regulation of developmental dynamics is the rule rather than the exception.

The endocrine system often mediates between the environment and gene expression. The nervous system actively responds to features of the environment, rapidly changing the hormone milieu within the individual—these hormones in turn alter gene expression, which in turn contributes to the maintenance or modification of phenotypic traits over multiple timescales.

Genes do not have a privileged role in accounting for phenotypes, because they are themselves regulated participants in the process of development.

Partitioning developmental outcomes into those attributable to the genes and those that are the result of the environment is not biologically plausible, despite the continued popularity of this approach in some quarters of biology and psychology.

The causes of development are always relational and distributed across factors internal and external to the developing individual. Modern developmental theory thus rejects attempts to functionally separate the organism and its environment.

Because of the multiplicity of levels, factors, and interactions and because of its dynamic and historical nature, development cannot have a predetermined trajectory. Development is contingent, context-sensitive, and open to multiple outcomes as well as multiple paths to the same outcome.

Introduction

Development is the fundamental phenomenon of biology. It is also one of its most challenging problems. What greater mystery could there be than the growth of something as complex as a human, made up of trillions of cells, from a relatively simple and formless tiny egg? The biologist E. S. Russell (1930 , p. 1) captured this enduring fascination with development over 75 years ago:

The general problem of development is without question one of the most difficult and intriguing in the whole field of knowledge. That from a minute germ of relatively simple structure there should gradually build up, by a series of processes beautifully coordinated in space and time, the complex organization of the adult is a fact that has never ceased to excite the wonder of mankind. It has provided a constant challenge to the intellect of man, and many and various have been the theories invented to explain it.

For most of the last century the standard theoretical interpretation of the remarkable transformation from egg to adult was decidedly reductionistic—genes were thought to directly control the process of development. From this view of development, the instructions for building organisms are present in their genes, and genes are also the exclusive means by which these instructions are transmitted from one generation to the next. Development was thus widely characterized as the process by which genotypic specification is translated into the traits and qualities of individuals, including their anatomy, physiology, and behavior. This genocentric view dominated the biological sciences during most of the twentieth century, prompting the formulation and promotion of the “Modern Synthesis” of evolutionary biology (an attempt to integrate Darwin’s theory of evolution with Mendel’s theory of genetics) during the first half of the century and facilitating significant advances in genetics and molecular and cellular biology in the second half of the century.

Despite these advances and despite the enormous scientific resources invested in the Human Genome Project to produce sequence information for the complete genome of humans, over the past several decades it has become increasingly clear to a majority of biologists and psychologists that attempting to understand development simply in terms of genes is implausible and cannot succeed. Evidence available from research in genetics, molecular and cellular biology, developmental biology, psychobiology, and the neurosciences has converged to suggest a very different view of development from the gene-centered explanations that dominated the biological sciences for most of the last century. This new view of development recognizes that genes are certainly a fundamental component of the developmental process but also indicates that their role in determining form and behavior has been vastly overstated, particularly in the popular media, where metaphors of “genetic programs,” “innate cognitive modules,” and “genes for traits” are still common.

Although the many mysteries involved in the processes of physical, behavioral, and psychological development are far from being solved, we now know that these processes involve widely distributed dynamics occurring across many levels of the individual organism and its local environment or ecology. Scientists working with species as divergent as fruit flies, cowbirds, rats, rhesus macaques, and humans have provided converging evidence that the development of any physical or behavioral trait is the result of a complex web of coactions among the individual’s genes, molecular interactions within and across cells, and the nature of the physical, biological, and social environments in which the individual develops (see Coen, 1999 ; Jablonka & Lamb, 1995 , 2005 ; Neumann-Held & Rehmann-Sutter, 2006 ; Noble, 2006 ). This “systems” approach to understanding gene activity is fundamentally different from the way the gene was characterized for most the last century and presents a significant challenge to the traditional gene-centered views of phenotypic development still evident in many quarters of biology and psychology.

The growing appreciation of the dynamic and contingent nature of development has led investigators working in biology, psychology, and philosophy to reconsider the established notion of genes as the primary cause of development, thereby opening the door to research programs focused on identifying how genetic and nongenetic factors act together to guide and constrain the process of development and its outcomes. This focus on the dynamics of development is often referred to as a “probabilistic epigenetic” or a “developmental systems” approach (e.g., Gottlieb, 1997 , 2007 ; Jablonka & Lamb, 2002 ; Oyama, 1985 ; Rollo, 1995 ; van der Weele, 1999 ). The consequences of this broader-based approach to understanding the ways and means of development are considerable and far-reaching, leading researchers to rethink the roles of activity and experience in development, the nature and extent of heredity, the origins of variability, and the relevance of development to evolution.

To situate and make sense of the revolutionary shift in thinking about the nature of development and heredity currently under way across the biological sciences requires looking back on conceptual insights and empirical discoveries made in a number of scientific disciplines over past several hundred years. As historians are quick to point out, all revolutions have their origins in the past. That being said, I do not attempt to provide a formal historical account of the biological study of development in this chapter. The interested reader will have to look elsewhere for this (see Needham, 1959 , and Moore, 1993 , for excellent examples). Rather, my emphasis here is to provide a broad overview of the themes, theories, and ideas from the biological sciences that are contributing to the emerging conceptual framework of contemporary developmental science.

Like many topics of modern science, the origins of many of our ideas regarding these issues can be traced back to the ancient Greeks. Conceptions of development and heredity were at the forefront of ancient Greek thought and debate. As I briefly review below, these debates most often focused on how to best characterize and explain several obvious but nonetheless mysterious characteristics of development still not fully understood even today.

The Phenomenon of Development: Accounting for Stability and Variability

Throughout history people have marveled at the process of development and pondered the possible causes at play in the orderly progression from fertilized egg to adult. This enduring fascination resulted in some strange and even bizarre notions about development through the centuries, including the idea that life could be infused into nonliving organic matter (so-called spontaneous generation , a view that led some to propose that insects could arise from decaying matter and mice from piles of old rags) and appeals to an élan vital , some ethereal vital force that was thought to animate and direct the transformation of the embryo into an adult (see Gould, 1977 , and Mayr, 1982 , for additional examples). As recently as 100 years ago, many biologists and psychologists thought that “ontogeny recapitulates phylogeny.” This view held that all organisms actually repeat the changes in embryonic form evolved by their evolutionary ancestors during their own individual development (see Haeckel, 1905 , and Hall, 1904 , for classic examples from biology and psychology respectively).

While these unusual ideas about development were gradually abandoned as a result of the technical, empirical, and conceptual advances realized over the past several centuries, much about the process of development remains unknown or poorly understood. Indeed, one can argue that development remains one of the most fascinating and least understood processes in all of biology. This is readily seen when one attempts to precisely define “development.” The standard definition of development as a “gradual unfolding” provided in most dictionaries implies that the traits and qualities of individuals somehow emerge from inherent essences or programs provided at conception. This “unfolding” metaphor of development is widespread across cultures ( desarrollo in Spanish, Entwicklung in German) and is also evident across the historical record, dating back at least as far as the writings of the ancient Greeks. Despite its widespread application in biological and psychological theories of development, the metaphor of unfolding may now be seen to be thoroughly nondevelopmental, a perspective that posits the scripted appearance of already-existing form. The continued use of this metaphor over the course of the twentieth century served to overemphasize the importance of internal factors to development and slowed progress in identifying the distributed causal processes we now recognize to be necessary for the transformation from fertilized egg to functional adult.

Despite the difficulty of precisely defining development, several key characteristics of development have been noted and discussed since the time of the ancient Greeks. One of these attributes of development is the increasing differentiation of form and function in an organism’s journey from a single fertilized cell to an adult. Across the amazing diversity of animal species found on earth, the process of physical and behavioral development involves progression from a simpler to a more complex organization, repeatedly bringing into being structures and responses of the organism that were not there before. As the developmental psychologists Linda Smith and Esther Thelen put it, “development is about creating something more from something less” (2003, p. 343). This pattern of increasing complexity across individual development was noted by the ancient Greeks and came to be termed epigenesis . The notion of epigenesis, well represented in the writings of Aristotle, held that development was a gradual and successive process—new structures and functions appear in stages over the course of development as a result of the interaction of the various parts of the developing organism. Many centuries later the notion of epigenesis matured into the view that the fertilized egg contains a small number of discrete elements and that during development these elements interact to produce the much larger number of features that were not present before these interactions.

This view contrasted with an opposing framework of development also common among the ancients, preformationism. Preformationism held that all structures of the adult organism were already present in the fertilized egg and development was thus merely the growth of a preformed miniature. From this perspective, development did not involve an increase in overall complexity during the course of the individual’s lifetime, as all the parts and organs were present and in their proper form from the outset. The idea of strict preformationism is gone from scientific thinking about development, largely abandoned due to evidence provided by the efforts of nineteenth-century experimental embryologists, including Karl Ernst von Baer, Wilhelm Roux, and Hans Driesch. Their experimental observations combined to make clear that the progression from relatively simple egg to fully formed adult occurs in an amazing temporal and spatial coordination of processes and events, with one stage of complexity and organization leading to the next (see Gottlieb, 1992 , and Moore, 2002 , for historical overviews).

For example, a butterfly begins life as an egg, emerges as a caterpillar, and then undergoes a complete change in body form during pupal development, emerging as an adult butterfly. A monkey begins life as an egg, then reorganizes into a zygote, embryo, fetus, infant, juvenile, and eventually adult monkey. The specific details of how such orderly differentiation of form and function occurs are still not fully understood, but as I review in the sections that follow, a considerable amount is now known about how genes, cells, hormones, and a wide array of physical, biological, and social factors contribute to this remarkable process of differentiation.

A second fundamental characteristic of development is the fact that on the journey from fertilized egg to adult, all organisms pass through cycles of change and reorganization that result in their closely resembling other members of their species. For example, we all know and expect that the fertilized eggs of a chicken will produce more chickens (and not turkeys) and those of a mouse will produce other mice (and not hamsters). Further, readily observable resemblances in traits and qualities are typically seen between parents and their offspring. The fact that offspring come to closely resemble their parents both physically and often behaviorally has, of course, been appreciated throughout human history. For example, the successful domestication of plants and animals depended on the application of the common insight that “like begets like” to selective breeding for favored traits and qualities.

How to account for this stability of form and behavior from one generation to the next has engaged some of the great scientific minds from Aristotle to the present. For example, Charles Darwin struggled with how to account for the fact that “like begets like” and eventually settled on the notion of pangenesis to explain the inheritance of traits and the guidance of development across generations. Darwin’s theory of pangenesis was based on ideas originally articulated by the ancient Greeks and held that as the cells of the body grow and divide they release very small invisible particles, called gemmules, that disperse throughout the developing organism’s body. As the organism matures these very small particles, contributed by different cells from all parts of the body, become concentrated in the sex cells (egg and sperm). At reproduction the organism’s accumulated gemmules are passed on to its offspring, thereby allowing the fertilized embryo to contain the basic cellular ingredients for the specific features of all its organs and body parts. The transmission of gemmules across generations was thus thought to explain how specific characteristics of the parents were transmitted to their offspring and thus how offspring come to closely resemble their parents.

Darwin’s notion of pangenesis was widely debated over several decades and eventually shown to be mistaken. The rise of the science of genetics in the early years of the twentieth century and the later growth of molecular and cellular biology provided a compelling alternative explanation for the observed stability of traits and characteristics from one generation to the next. A host of remarkable discoveries about genes, DNA structure, and RNA transcription during the past half-century led to the widely accepted view, still held in many quarters, that heredity was exclusively gene-based . This narrow framework further contributed to the entrenchment of genocentric views of both development and evolution that had gradually come to dominate thinking in the biological sciences during the first half of the last century. However, the dynamics of development that allow for the stability of form and function observed within and across generations in any given species has been shown in recent years to be much more environmentally contingent and much more probabilistic than gene-centered accounts would allow. As I review below, how internal and external factors and their interrelations contribute to species-typical development turns out to be much more complicated than was acknowledged by the reductionistic views of heredity and development widely promoted during the last century.

A third fundamental feature of development is the inverse of the second. Just as individuals of a species show remarkable stability of form and behavior from one generation to the next, they also show variations in these qualities. Indeed, every individual of a group or population is unique. This variation across individuals of a species was a crucial component of Darwin’s notion of evolution by natural selection. He realized that for natural selection to act, individuals must vary in terms of their anatomy, physiology, or behavior. For Darwin, such variations provided the opportunity for natural selection to filter out those anatomical structures, physiological capabilities, and behavioral forms that are less successful and to promote those that offer some reproductive advantage, thereby providing the engine for evolutionary change.

The sources of such phenotypic variations were not well understood in nineteenth- or much of twentieth-century biology. Darwin himself admitted that “our ignorance of the laws of variation is profound” (1859, p. 167). It took biologists and psychologists the better part of the twentieth century to start considering phenotypic variation as an important area of study rather than as noise or nuisance to experimental designs. Only recently have developmental and evolutionary scientists focused their research efforts on this topic in a more systematic manner (see West-Eberhard, 2003 , for a masterful review), and efforts in this regard are being made by biologists and psychologists working in formally diverse areas of research such as cellular and molecular biology, developmental biology, evolutionary theory, ecology, and comparative and developmental psychology. As we gain a deeper appreciation of the importance of the process of development to the production of phenotypic variation, new questions are being raised about how to think about the sources of individual differences and about the nature and extent of the links between developmental and evolutionary change (e.g., Bjorklund, 2006 ; Johnston & Gottlieb, 1990 ; Oyama, 1985 ; West-Eberhard, 2003 ), topics I return to later in this chapter.

Any comprehensive theory of biological development must ultimately account for each of these three fundamental features, namely the emergence of complexity of organization by differentiation , the stability of form and function across generations , and the origin and range of variability across individuals of a species . Attempts at this intellectual synthesis have engaged and frustrated scientists for centuries. Indeed, much of the content of eighteenth- and nineteenth-century theorizing about development and heredity focused on explaining the possible mechanisms for these three developmental phenomena (see Depew & Weber, 1995 ; Mayr, 1982 ; Sapp, 2003 , for overviews). In the first half of the twentieth century, research and theory from the emerging fields of cellular biology and population genetics persuaded many biologists to adopt a decidedly bottom-up approach to account for the similarities and differences observed across individuals. Genes were widely held to both orchestrate an organism’s growth and development and provide for the intergenerational stability and variability of traits and qualities observed within species (see Keller, 2000 , and Sapp, 2003 , for reviews). The biologist J. T. Bonner (1987 , p. 715) succinctly captured this instructionist view of development: “we all recognize that in the fertilized egg there is a set of genes…and that these genes give instructions that ultimately produce a complex adult.”

This genocentric framework was the established view for many decades in evolutionary biology ( Dobzhansky, 1937 ; Fisher, 1930 ; Mayr, 1942 ; Simpson, 1944 ; Williams, 1966 ), molecular and cellular biology ( Bonner, 1965 ; Gehring, 1998 ; Jacob, 1976 ), and ethology and animal behavior ( Hamilton, 1964 ; Lorenz, 1965 ; Wilson, 1975 ). Within the behavioral sciences, this framework fostered widespread application of the notion of “innate” or “instinctive” behavior, patterns of action thought to be genetically determined and hardwired in the organism at conception. Application of the genocentric framework also fostered the growth and popularity of fields such as sociobiology, behavioral genetics, and evolutionary psychology ( Laland & Brown, 2002 ). The widespread acceptance of a gene-centered view of development and heredity across the life sciences over the past 60 years can be traced in large part to the influential research and writings of experimental biologists and embryologists of the nineteenth and early twentieth century. As I review in the next section , their findings and conclusions provided a collective framework that set the course for how most scientists mistakenly came to characterize what development is, how it works, and what its importance might be to evolution.

Establishing the Genocentric View of Development: The Notion of Prespecification

The science of biology emerged as a distinct field of inquiry during the first half of the nineteenth century. The discoveries of experimental pioneers like Karl Ernst von Baer, Theodor Schwann, Matthais Schleiden, and Johannes Muller contributed to the eventual formalization of many of the subdisciplines of biology still with us today (see Depew & Weber, 1995 ; Mayr, 1997 ). For example, von Baer’s discovery of the mammalian egg in 1827 allowed him to experimentally confirm an idea proposed by Aristotle some 2,100 years earlier, that the animal embryo develops from an undifferentiated state to a highly differentiated one. His detailed descriptions of embryological sequences of fish, birds, and mammals provided an initial map of the process of differentiation, carefully documenting that development proceeds from the general to the more specific. von Baer’s groundbreaking observations set the stage for the subsequent growth of experimental embryology and developmental biology. In the next decade, Schleiden in 1838 and Schwann in 1839 independently demonstrated that all organisms are composed of cells. Moreover, they were able to show that cells share many common features whether they derive from a plant or an animal. Their discoveries provided the basic foundation for modern cellular biology and suggested that the division and multiplication of cells might help explain the mystery of differentiation that had perplexed students of development for centuries.

A few decades later, the experimental work of the Austrian monk Gregor Mendel would eventually set in motion the conceptual and empirical foundation for what the historian of science Evelyn Fox Keller (2000) has termed “the century of the gene.” Mendel’s research on the laws of inheritance in garden peas (resurrected some four decades after its initial publication in 1865) suggested to him that heredity came packaged in discrete units that could be combined in predictable ways. Mendel proposed that each of these discrete units or factors was associated with a particular phenotypic trait or character, providing a one-to-one correspondence between heredity factors and the structure or properties of the organism. Further, he proposed that each character was represented in the fertilized egg by two factors, one derived from the father and the other from the mother. These factors (later termed “genes”) were thought to be self-contained packets of inheritance, passed on from generation to generation.

Mendel’s research and theorizing provided a basis for forging a conceptual dichotomy between the characters and qualities of individual organisms and the factors or “units” of heredity responsible for these characters that passed from parent to offspring in the process of reproduction. This dichotomy was eventually formalized into the terms phenotype (the appearance and behavior of the individual organism) and genotype (the total repertoire of hereditary units acquired at conception) during the early years of the twentieth century ( Johannsen, 1911 ). During this time a group of influential biologists, including Hugo de Vries, William Bateson, and Thomas Hunt Morgan, were busy using Mendel’s proposed principles to solidify the view that heredity (and the resulting stability and variability of traits and qualities observed across generations) involved the passing on of discrete internal factors situated somewhere in the structure of fertilized cells. These internal factors were termed “genes” by the Danish botanist Wilhelm Johannsen in 1909 and eventually came to be seen by many biologists as the physical units that determined the development of the physical appearance and behavioral characteristics of all organisms. These units of heredity were assumed to be passed on in reproduction, thereby also determining the phenotype of the next generation of developing organisms.

Many (but certainly not all) geneticists and developmental biologists of the early twentieth century also assumed that the genes were somehow protected from any effects arising from the experience of the organism during its own lifetime. The notion of a barrier between the genes and an individual’s experiences is usually credited to the influential writings of the nineteenth-century German biologist August Weismann. Like Darwin, Weismann wrote widely on heredity, development, and evolution. Like Darwin, he thought that heredity involved little particles transmitted from parent to offspring, which he termed “determinants.” Unlike Darwin, however, Weismann believed that the “germ plasm” containing these particles was sequestered from any influences arising during an individual’s development. He reached this conclusion on both ideological and empirical grounds.

Ideologically, Weismann was reacting to the then-popular notion of the inheritance of acquired characteristics, a view of development derived from Jean Baptiste de Lamark’s theory of evolutionary transformations. Lamark’s writings on the mechanisms of heredity during the eighteenth century had influenced several generations of scientists concerned with evolution, including Darwin. According to Lamark, the activities of organisms in response to the specific demands of their environment resulted in adaptive changes in anatomy, physiology, or behavior, some of which could be passed on to their offspring (for example, the long necks of giraffes). Darwin and many of his contemporaries accepted this view of development and evolution. Weismann, on the other hand, opposed this “soft” view of inheritance and set out to disprove it. In 1883 Weismann published On Heredity , which summarized his theory of what came to be known as “hard inheritance.” According to this view, germ cells are not affected by somatic cells, thereby effectively eliminating the mechanism for the inheritance of acquired characteristics. In what many considered one of the most influential experiments of the time, Weismann demonstrated that mice whose tails had been cut off in 19 successive generations did not subsequently produce tailless rat pups. Indeed, each generation was born with a full-length tail, and in the final generation of mice (the 19th) their tails were as long as those measured on the first generation of breeding. He thus concluded that traits or qualities acquired by individuals during their lifetime could not be passed on to their offspring. It would take nearly 100 years for developmental and evolutionary biologists to recognize the significance of the link between development and heredity and thereby reframe Weismann’s question as not whether adult traits (chopped-off tails) could be inherited by offspring, but whether modifications of the developmental process could be reliably transmitted from one generation to the next.

Weismann argued that there was a complete separation of the germ plasm (what in later years would be termed the genotype) from its expression in the phenotype. As a result, only changes in the “germ line” (contained in the sperm and egg) could contribute to heredity and ultimately to evolution ( Weismann, 1889 ). From this view, the fertilized egg contains all the necessary information for the development of the organism and this information remains insulated from any environmental influences occurring during the individual’s own lifetime. Weismann argued that this was necessarily the case because the separation of the germ cells from all other cells of the body (what he called the “somatic line”) occurred so early in the individual’s development that what happened to somatic cells over the individual’s ontogeny had no opportunity to effect the makeup of the germ cells. Changes in genes and any resulting evolutionary change would have to come from somewhere other than an organism’s life experience.

The correctness of this view has been seriously questioned within the biological sciences over the past several decades (see Gottlieb, 1998 ; Jablonka & Lamb, 1995 , 2005 ; Laubichler & Maienschein, 2007 , for examples), but it is certainly the case that Weismann’s narrow conception of the nature and extent of heredity had an enormous influence on the direction of theoretical biology for several generations. Indeed, his assumption of internal factors determining development (and the minimal importance or influence of the environment) is still common in some quarters of contemporary biological and psychological thought. As Griesemer (2002) has pointed out, Weismann’s views provided the basic causal structure used to articulate ideas about genotype and phenotype, heredity and development, and evolution and selection for most of the twentieth century.

Formalizing the Notion of Prespecified Development: The “Modern Synthesis” of Biology

The acceptance of Weismann’s perspective on the nature of heredity and development during the early decades of the twentieth century resulted in developmental issues becoming more and more divorced from evolutionary issues within biology. If genes contained all the necessary information for phenotypic traits, and if circumstances during individual development could not directly influence the traits or characteristics of offspring, then any role or influence of development in evolution had to be minimal. As the evolutionary biologist Maynard-Smith (1982 , p. 6) put it: “one consequence of Weismann’s concept of the separation of the germ line and soma was to make it possible to understand genetics and hence evolution, without understanding development.”

Following the rediscovery of Mendel’s work in 1900 and the growing influence of Mendelian genetics during the next several decades, evolutionary biology came to distance itself from its earlier concerns with embryology and developmental biology and embrace the new science of population genetics (see Gilbert, 1994 , and Gottlieb, 1992 , for overviews). Unlike experimental embryology, population genetics was not focused on the development of the individual organism. Rather, it focused on organisms as members of a breeding population and how best to calculate the probabilities of changing gene frequencies in this population of breeding organisms under this or that set of circumstances over generations. In particular, population geneticists were interested in understanding the role of genetic mutation, genetic recombination, and selection in the changes in gene frequencies found within a population and in understanding how these genetic changes contributed to evolutionary change. Population genetics assumed any genetic variation to be random and stochastic, and not directly influenced by the environment. This approach to heredity and evolution concentrated on the traits of adults in populations and virtually ignored questions about how these traits were actually realized during individual development (see Moore, 2008 ).

The shift in focus away from development, exemplified by the career path of the pioneering geneticist Thomas Hunt Morgan (see Allen, 1978 ; Amundson, 2005 ), was solidified by the Modern Synthesis of evolutionary biology crafted in the 1930–1940s. This shift resulted in the promotion of a very narrow definition of evolution as “a change in the genetic composition of populations” ( Dobzhansky, 1937 ). This definition explicitly assumed that population genetics by itself could provide a complete theory of evolution. Following a framework most clearly articulated by R. A. Fisher (1930) , a species or a population became a collection of discrete Mendelian genes existing in different frequencies (so-called “gene pools”) that were the objects of evolutionary change. Evolution was assumed to be changes in genes rather than changes in developmental processes. This very narrow perspective on evolution was widely embraced by several generations of biologists and was grounded on three widely held underlying assumptions regarding development and heredity:

Instructions for building organisms reside in their genes.

Genes are the exclusive means by which these instructions are faithfully transmitted from one generation to the next.

There is no meaningful feedback from the environment or the experience of the organism to its genes.

These three assumptions fit neatly within the conceptual framework of population genetics. The architects of what came to be known as the “Modern Synthesis” (including the biologists Theodore Dobzhansky, Julian Huxley, Ernst Mayr, and George Gaylord Simpson) saw no need to integrate a concern with development into their collective attempts to synthesize the tenets of Darwinism, Weismannism, and Mendelism into the Modern Synthesis of evolutionary biology. The population geneticist Bruce Wallace (1986 , p. 158) captured the logic of the then-popular view that development was irrelevant to evolution:

The development of the individual is governed by the developmental program which that individual has inherited from its parents. If the program is successful in producing an adult, reproducing individual, that program is transmitted to offspring who make up the subsequent generation. If not, the responsible program stops. The relative proportions of various genetic programs among the incoming germlines and the continuing germlines need not be identical: the lowest level of evolution consists, then, of changes in the frequencies of genetic programs—or stated more simply, of gene frequencies.

The Modern Synthesis was able to effectively sidestep concerns with development by arguing that there were two relatively independent classes of causal factors responsible for an individual’s phenotypic traits: (1) ultimate causes , those that derive from internal or intrinsic factors (e.g., genes), molded over evolutionary time by natural selection, and (2) proximate causes , everything else that interacts with these internal factors during development to provide the materials or experiences necessary to trigger the expression of form and function thought to be encoded in the genes (e.g., the environment). This causal dichotomy for explaining developmental outcomes was further grounded on the assumption that development is primarily internally determined, set on course at conception by genetic programs ( ultimate causes ) that had been designed and selected over evolutionary time. In contrast, proximate causes were defined as those factors involved in “decoding the genetic program” (see Mayr, 1974 ). Developmental factors were thus seen as proximate causes, making development essentially irrelevant to the understanding of heredity and evolution ( Lickliter & Berry, 1990 ).

Watson and Crick’s discovery of the structure and function of DNA in 1953 served to reaffirm the genocentric position of the Modern Synthesis—if genes are DNA, and copying errors from DNA to RNA to protein are the source of genetic variation, then evolution must indeed be “changes in gene frequencies in populations.” Development was thus increasingly viewed as merely the reading out of genetic programs that were assumed to be the products of natural selection. This perspective had at its core a fundamental underlying premise that went unquestioned by most researchers and theorists of this period: the bodily forms, physiological processes, and behavioral dispositions of organisms can be specified in advance of the organism’s development. This postulate is at the heart of the genocentric view of development that dominated biological thought over the last century and that remains prominent in some quarters of biology and psychology. However, the assumption of prespecification is a profoundly nondevelopmental view, very much in keeping with the preformationistic dictionary definition of development as a “gradual unfolding” of what is presumably already there. Adult traits are seen to be the result of genetic instructions or programs, with little concern for the intervening resources, relations, and causes that construct the adult from the zygote. This view has several serious shortcomings, not the least being that it assumes as a given the developmental outcomes that actually require a causal developmental analysis ( Gottlieb, 1997 ; Kuo, 1967 ).

Although enormously influential in biological thinking over the last half-century, the notion of prespecified traits is increasingly recognized as not up to the task of making sense of the dynamics of the developmental process and its outcomes. As the astute biologist E. S. Russell (1930) noted more than 75 years ago, the fault of all preformistic, predetermined, or prespecified theories of development is that they translate the future possibilities of development into “material” predispositions such as gemmules, determinants, or genes. These potentialities are, however, purely virtual and conceptual, an illusion created by a bad metaphor. The appearance or realization of supposedly specified traits is entirely dependent on the distributed resources, relations, and interactions that make up the process of development.

Russell’s insight has received substantial empirical support from biology and psychology in recent years and is forcing a conceptual revolution regarding how development, heredity, and evolution are characterized within the life sciences (e.g., Griesemer, 2002 ; Oyama, Griffiths, & Gray, 2001 ; Robert, 2004 ; Strohman, 1997 ; West-Eberhard, 2003 ). Perhaps the most significant aspect of this conceptual shift centers on how we think about the roles of activity, experience, and context in the achievement, maintenance, and modification of phenotypic traits.

Coming to Terms with the Roles of Experience and Context in Development

Samuel Butler, the nineteenth-century British novelist, satirist, and amateur biologist, observed that “life is like giving a concert on the violin while learning to play the instrument.” Butler’s insight that individual ontogeny occurs “in the middle of things” points out a key feature of development that was generally ignored in most gene-centered views during the last century—development always involves the specific experiences, conditions, and contexts individuals encounter and take part in as they live their lives. The growing recognition that development is an historical process that is situated within and dependent on the experiences and activities of the organism represents a major shift in thinking from the dominant prespecified view of development of the last century. As we have seen, this view downplayed the importance of context and experience to the realization of species-typical anatomy, physiology, and behavior, in large part because such “proximate” factors were seen to simply fine-tune the “ultimate” or intrinsic factors thought to actually determine the phenotype.

Contrary to this genocentric view, it is now well documented that specific features of the environment of development, coacting with the organism’s genome and its products, can determine the sexual phenotype in some species of fish and reptiles, induce specific morphological changes that allow individuals to escape predation in several amphibian species, and bring on caste determination in a number of species of insects (see Gilbert, 2001 , for discussion and additional examples). Approaching phenotypic traits as outcomes that are generated in context, rather than expressed from a prespecified program, is transforming how developmental research is done in biology and psychology. In particular, recognition that development always takes place in some “experiential” context (where experience is defined broadly to include functional activity and the various stimulative aspects to which individuals are subject during prenatal and postnatal life) is fostering a more focused and explicit concern with the ecology of development, the immediate features and properties of an organism’s niche involved in guiding, facilitating, maintaining, and constraining the dynamics of the developmental process (see West, King, & White, 2003 ; West-Eberhard, 2003 ).

An example from the study of anatomical development serves to illustrate this shift in emphasis. The vertebrate limb has been a subject of study for more than 150 years. From the gene-centered view dominant over much of the last century, limb development was seen as a relatively straightforward affair—genes were thought to provide the specific instructions for the growth and development of bone and muscle, thereby accounting for the observed stability of form and function within a species and across generations. Recent evidence from developmental biology indicates otherwise. For example, we now know that in vertebrates the active movement of the embryo is required for the normal or species-typical development of bone, joints, muscles, tendons, and ligaments ( Müller, 2003 ). As a case in point, the fibular crest is a leg bone that connects the tibia to the fibula in most bird species. It allows the force of the iliofibularis muscle to pull directly from the femur bone to the tibia bone. This direct connection between the femur and tibia is important, as it allows the reduction in size of the femur bone seen in birds when compared to mammals. Developmental biologists have shown that when chicken embryos are prevented from moving within the egg during periods of their prenatal development, this bone fails to develop ( Müller & Steicher, 1989 ). In other words, embryonic movements are necessary to induce the development of bone in the chick embryo.

Under the normal conditions of prenatal development the bird embryo is subjected to ongoing stimulation from a host of factors, including gravity, thermal gradients, amnion contraction, maternal stimulation, and also self-stimulation of its own muscles, joints, and sensory systems as it moves and positions itself in the egg (or, in the case of the mammalian embryo, the uterus). For example, in the chick embryo the first muscle contractions are observable by the third day of incubation. The prenatal environment (and later the more complex postnatal environment) thus provides a range of stimulation and activity that turns out to be essential for normal anatomical, physiological, and behavioral development (see Gottlieb, 1997 , and Lickliter, 2005 , for behavioral examples). In the example of skeletal development, the use and exercise of the chick embryo’s leg turns out to influence gene expression, the activity of nerve cells and their processes, as well as the release of various neurochemical and endocrine secretions during prenatal development. All of these factors and their interactions are necessary resources for the normal development of the skeleton of the young bird, starting with the patterned deposition of cartilage-forming cells, the precursors of the bones ( Streicher & Müller, 1992 ).

The complex interactions between genes, gene products, and external influences involved in avian skeletal development illustrate a basic feature of the process of development only recently appreciated by most biologists and psychologists—what a gene does in terms of what it provides development depends on the expression and activity of other genes, as well as nongenetic factors internal and external to the organism. In other words, genes are not exempt from influences at other levels of analysis and are, in fact, dependent on them for initiating and terminating their activity. As a result, genetic and nongenetic factors cannot be meaningfully partitioned when accounting for developmental outcomes. A growing number of biologists are thus expanding the focus of their research attention to not only the internal features of the developing organism (genes, proteins, cells, hormones) but also the contributions of the varied physical, biological, and social resources available to the individual in its developmental context (diet, temperature, social interaction; see Gilbert, 2001 , 2005 ).

This complex web of interactions among genes, their products, and the internal and external environment is the particular focus of epigenetics , a rapidly growing field within the biological sciences that has been typically defined as the study of heritable changes in gene expression and function that cannot be explained by changes in DNA sequence ( Holliday, 1994 ; Richards, 2006 ) or more broadly as the study of how the environment can affect the genome of the individual during its development, as well as the development of its descendants, without change in the coding sequence of the genes ( Crews, 2008 ). Epigenetics includes the study of how patterns of gene expression are passed from one cell to its descendants, how gene expression changes during the differentiation of one cell type into another, and how environmental factors can modify how genes are expressed. As I briefly review below, accumulating evidence from epigenetic research is showing that, contrary to Weismann’s influential doctrine of the encapsulated genome, genetic activity is regularly influenced by neural, behavioral, and environmental events across the course of development (see Gottlieb, 1998 , and Jablonka & Lamb, 2005 , for multiple examples).

The Rise of Contemporary Epigenetics

Although only recently popular in terms of textbooks, journals, and conference proceedings, epigenetics is not a new topic to biology. The embryologist and geneticist Conrad Waddington described epigenetics early in the 1940s as the branch of biology that studies the causal interactions of genes with their environment that bring the phenotype into being ( Waddington, 1942 ; see also Waddington, 1957 ). Of course, the genetic, molecular, and cellular details of phenotypic development were poorly understood at that time. Indeed, in the first half of the twentieth century the gene was largely a theoretical concept without a physical identity ( Crews & McLachlan, 2006 ). Nevertheless, based in part on his experimental work with fruit flies, Waddington came to question the canonical view that there was a simple correspondence between genes and traits and proposed that only an understanding of the interaction of genes with each other and with the internal and external environment of the organism could account for phenotypic development. Waddington was advocating a new conceptual framework for the study of development and evolution, one that emphasized changes in what he termed “developmental systems.” From this view, the contribution of the genome always depends on the influence of the features of its surrounding contexts, beginning with the cytoplasmic environment provided by the mother’s egg at conception. Waddington’s efforts to integrate genetics, development, and evolution were well ahead of the prevailing consensus of his time and were motivated by what he viewed as the inability of population genetics to provide a workable model of the operation of genes in development and evolution ( Hall, 2001 ).

During this same period, developmental psychobiologists concerned with the development of behavior were also emphasizing the importance of understanding how genetic, organismic, and environmental factors coact to generate and modify phenotypes. In his cogent critique of Konrad Lorenz’s view of innate behavior, Lehrman (1953 , p. 135) argued that “the problem of development is the problem of new structures and activity patterns from the resolution of the interaction of existing structures and patterns, within the organism and its internal environment, and between the organism and its outer environment.” Lehrman’s experimental work on the reproductive behavior of ring doves provided an elegant example of the rich network of temporally specific transactions between internal and external factors involved in the initiation and maintenance of courtship, nest-building, egg-laying, incubation, and parental care. His findings argued against the genetic specification of species-typical behavior and suggested a more dynamic and distributed view of developmental causality. From a psychobiological point of view, development is best accounted for by the bidirectional traffic of genetic, neural, behavioral, environmental, and social factors operating across what Gottlieb (1970 , 1976 ) termed “the developmental manifold.”

The dynamic, epigenetic frameworks of development outlined by Waddington, Lehrman, Gottlieb, and a handful of other biologists and psychologists working in the middle of the twentieth century (e.g., Kuo, 1967 ; Løvtrup, 1974 ; Rosenblatt, 1970 ; Schneirla, 1966 ; Tobach, 1970 ) were relatively ignored across most of biology for the next several decades. However, a growing body of evidence drawn from genetics, cellular and developmental biology, neuroscience, and developmental psychology has recently converged to support the validity of the probabilistic epigenetic framework (e.g., Bjorklund, 2006 ; Davidson, 2001 ; Gottlieb, Wahlsten, & Lickliter, 2006 ; Jablonka & Lamb, 1995 ; Michel & Moore, 1995 ; Rakyan & Beck, 2006 ; Szyf, Weaver, & Meaney, 2007 ). While it is certainly the case that our knowledge of the intricacies and contingencies of gene–environment relations are still piecemeal and incomplete, findings from epigenetic research are beginning to document the intricate bidirectional regulatory networks involved in the developmental process, as well as pointing to the need to revise several enduring ideas and principles regarding development and heredity over the last century. These ideas and principles include the notions that genes contain specific programs or instructions for building organisms ( predetermined epigenesis ), that genes are the exclusive vehicle by which these instructions are reliably transmitted from one generation to the next ( heredity as gene transmission ), and that there can be no meaningful feedback from the environment or the experience of the organism to the genes ( genetic encapsulation ). As I explore below, these traditional principles of development and heredity are all being challenged by findings from contemporary epigenetic research.

The Bidirectionality of the Organism–Environment System

Recent discoveries from epigenetics have made clear that gene expression is determined by the developmental system as a whole, with feedback loops between genes, cells, organs, body, and environment. As the philosopher Richard Burian (2005 , p. 177) put it, “the context-dependence of the effects of nucleotide sequences entails that what a sequence-defined gene does cannot be understood except by placing it in the context of the higher-order organizations of the particular organisms in which it is located and in the particular environments in which those organisms live.” Genetic studies with human identical twins have provided a dramatic demonstration of this insight, revealing the extent to which lifestyle and age can influence gene activity and expression.

Researchers working in Spain found that 35% of 80 sets of identical twins had significant differences in their DNA methylation and histone modification profiles, useful markers of patterns of gene activity and expression ( Fraga et al., 2005 ). The DNA in every cell nucleus is wound around proteins called histones and must be unwound to be transcribed. Modification of this packaging makes genes more or less available to the cell’s chemical signals that determine whether the gene is expressed or silenced. Research has shown that genes on loosely packed DNA are more likely to be expressed than are genes on those that are more tightly wound. This is the case because the looser the packing, the easier it is for various molecules to gain access to genes and initiate their activation; the denser the packing, the more difficult it is for molecules to gain access and the more genes are effectively silenced. DNA methylation is the addition of a methyl group to cytosine bases in the DNA sequence, which interferes with the chemical signals that allow a gene to be activated, thereby also effectively silencing the gene. Although DNA methylation is only one of several epigenetic factors than can alter gene expression (e.g., transposon activity; micro-RNA interference; x-chromosome inactivation; genomic imprinting), it is particularly stable, and patterns of methylation can be maintained after cell division ( Razin, 1998 ). Interestingly, twins who spent less time together during their lives or who had different medical histories showed the greatest differences in their methylation and histone profiles. Further, the older the twin pair, the more different they were when compared to younger twins. For example, a 50-year-old pair of twins had four times as many differently expressed genes as did a 3-year-old pair ( Fraga et al., 2005 ). These findings illustrate the significant influence of environmental and experiential factors on gene activity over the lifespan. They also help explain how genetically identical individuals can differ in their phenotypic traits and qualities, a common observation of the families and friends of identical twins.

Work with mice has also provided a dramatic demonstration of this phenomenon ( Waterland & Jirtle, 2003 ). A particular strain of genetically identical mice has what is called the “agouti” allele. The expression of this allele typically results in a yellow coat color and a strong tendency toward obesity and diabetes. However, when agouti females were fed before and during pregnancy a diet high in folic acid and vitamin B12 (which are both high in methyl donors), their methyl-rich diet effectively silenced the “agouti” allele in their developing embryos. Females fed on the altered diet subsequently gave birth to mostly thin, brown-furred pups, whereas control mice gave birth to pups that were mostly fat and yellow. These fat pups in turn had higher susceptibility to obesity, diabetes, and cancer as adults. Importantly, these results were observed in genetically identical mice.

Differences in maternal behavior have been shown to have epigenetic effects similar to those produced by differences in diet (see Zhang et al., 2006 , for a review). For example, the variation in licking, grooming, and nursing styles that female rats display toward their pups has long been known to affect the behavior and stress response of the offspring, and these changes have recently been tied in part to changes in DNA methylation and histone acetylation in the pup’s hippocampus ( Weaver et al., 2004 ). Rat pups that receive less licking and grooming following birth have reduced DNA methylation and histone acetylation at a glucocorticoid receptor gene in their hippocampus, resulting in an increased stress response as adults. In contrast, pups that receive increased licking and higher-quality maternal care following birth have increased DNA methylation and histone acetylation in their hippocampus and as a result release less cortisol when startled or placed in new surroundings as adults. Cross-fostering studies have confirmed that these changes are mediated by variations in maternal care received by pups following birth and remain after weaning and into adulthood, even persisting across subsequent generations ( Francis et al., 1999 ).

If the stimulative context in which an individual develops influences its physiology and behavior as an adult, it follows that the activity of the nervous system would also be affected. Epigenetic gene regulation has also been shown to be involved in the development and function of the nervous system, including cell differentiation, neural plasticity, learning, and memory (see Feng, Fouse, & Fan, 2007 , for a review). For example, there is increasing evidence that the regulation of chromatin structure through histone acetylation and DNA methylation patterns mediates long-lasting synaptic changes in the context of learning and memory ( Levenson & Sweatt, 2005 ). Epigenetic mechanisms provide a means of dynamic gene regulation, allowing the nervous system to make long-lasting changes at the level of neural circuitry and neurotransmission as a result of experience. Long-lasting changes in synaptic plasticity are one of the key mechanisms underlying learning and memory, and evidence suggests that histone modification and chromatin remodeling are involved in the activation or inhibition of memory storage-related gene expression ( Guan et al., 2002 ). Such epigenetic changes in synaptic structure and function likely represent a key mechanism for regulating both neuronal and behavioral plasticity.

It has long been known that exposure to an enriched environment can cause changes in brain function, including enhanced learning and memory, as well as changes in brain structure, including enhanced dendritic branching, synaptic density, and neurogenesis (see Nithianantharajah & Hannan, 2006 , for a review). Recent research with mice has documented that juveniles receiving just 2 weeks of an enriched environment exhibit a novel cell-signaling cascade that contributes to the induction of long-term potentiation (LTP), known to be important for learning and memory ( Li, Tian, Hartley, & Feig, 2006 ). Even more remarkable is the finding that this enhancement of LTP is seen in their offspring, even if the offspring have never experienced an enriched environment ( Arai, Li, Hartley, & Feig, 2009 ). Moreover, cross-fostering revealed that once the offspring are born, the enrichment history of the female mothering them does not influence the mechanism of LTP, indicating that the transgenerational inheritance of the effect of enriched rearing occurs before birth. Taken together, these findings demonstrate the power of environmental factors to modulate the signaling network that promotes LTP in the hippocampus and indicate that this modulation can in some cases persist across generations. The notion that the effects of enriched rearing on the mother can be passed on to her offspring during prenatal development has also been supported by behavioral work by Kiyono and colleagues, who showed that exposing pregnant rats to an enriched environment enhanced the maze-learning abilities of their offspring, even when the offspring were reared by nonenriched foster mothers ( Kiyono, Seo, Shibagaki, & Inouye, 1985 ).

David Crews and his colleagues have provided evidence that epigenetic factors can also affect reproductive behavior transgenerationally. When pregnant rats were exposed only once to a fungicide toxin (vinclozolin, used on a variety of agricultural products) that is known to alter DNA methylation, this exposure affected mate choice behavior for three subsequent generations. Previous studies had shown that male offspring of mothers exposed to vinclozolin were usually sterile or produced sperm with impaired mobility. Further, in each generation males whose ancestor had been exposed to the toxin showed an increase in cancer and prostate and kidney disease. When tested in a partner preferences paradigm, females that were three generations removed from the one-time toxin exposure discriminated and preferred males who did not have a history of toxin exposure; males did not exhibit such a preference when tested with exposed females ( Crews et al., 2007 ). Modification of DNA methylation by toxin exposure thus altered mate choice not only in the exposed generation, but also in several generations removed from the one-time exposure.

Taken together, these various demonstrations of the bidirectional traffic among context, experience, and gene expression highlight the fact that environmental and genetic influences are coactors in phenotypic change, including that which extends across generations. In addition to their genes, organisms inherit a wealth of developmental resources, and as the rodent maternal behavior work makes clear, this typically includes a stimulative environment containing parents and peers as well as the varied provisions of their ecological and social niche. These features of the developmental ecology can extend across generations and contribute to both the stability and the variation in phenotypic outcomes that researchers in biology and psychology seek to understand ( Gilbert & Epel, 2009 ; Lickliter & Harshaw, 2010 ).

The findings now available from epigenetic research suggest that environmental influences during human prenatal development, as well as infancy and childhood, might affect phenotypic outcomes in adulthood. Cacioppo and colleagues (2000) , Harper (2005) , Moffitt (2005) , and Shanahan and Hofer (2005) have provided overviews of studies that are beginning to identify relations between the features of the physical, nutritional, behavioral, and social environments encountered and gene expression in human development and discuss various ways in which these studies can be extended. For example, the nature of the rearing environment can apparently mediate the influence of a polymorphism in the gene involved in the production of the neurotransmitter metabolizing enzyme monoamine oxidase A (MAOA). MAOA is necessary for dopamine and serotonin neurotransmission and its level of activity has been implicated in the etiology of conduct disorder, violent offenses, disposition toward violence, and antisocial personality disorder ( Caspi et al., 2002 , 2003 ). In a sample of 1,000 New Zealanders who were assessed from age 3 through 26, men who had been maltreated as children were more likely to exhibit violent or antisocial behavior as adults. This, of course, is not unexpected. However, further analyses linked these behaviors to MAOA activity. A substantial number of men went on to exhibit some form of violent or antisocial behavior if they had the short form of the MAOA polymorphism and they were reared under conditions of severe maltreatment. Male children reared without maltreatment or with only “probable” maltreatment were unlikely to become violent or antisocial even if they had the short MAOA polymorphism. In contrast, having the long form of the MAOA polymorphism reduced the probability of the development of violent or antisocial behavior, even under conditions of severe childhood maltreatment (see Fox, Hane, & Pine, 2007 , for a similar example using stress reactivity and anxiety). Exactly how early life experience interacts with gene expression and MAOA activity in humans remains to be determined, but it is clear that the process would be poorly understood by focusing solely on genes.

Research has also begun to address possible transgenerational effects of early experience in humans. A study by Pembrey and colleagues ( Pembrey et al., 2006 ), using the Avon Longitudinal Study of Parents and Children, explored the effects of food supply on offspring and grandchild mortality risk ratios. They found that the paternal grandfather’s food supply (based on estimates from local harvest and food price records) was linked to the mortality risk ratios of grandsons, whereas the paternal grandmother’s food supply was linked with granddaughters’ mortality risk ratios. Importantly, these transgenerational effects were seen only when low food supply exposure occurred before the prepubertal periods of both grandparents, or during the fetal development of the grandmother. Such findings suggest that the environment can induce epigenetic changes in the sex chromosomes, which are then passed on to offspring (and in turn their offspring). In other words, sex chromosomes can apparently be marked across generations according to the parent of origin, suggesting a mechanism for the intergenerational transmission of modifications of gene expression via the gametes. Of course, our understanding of the ways and means of epigenetic variation and inheritance is still in its infancy and additional longitudinal analyses and research are needed to further investigate this intriguing possibility. Existing evidence does, however, demonstrate that the determinants of individual differences include environmental events that affected prior generations ( Bateson & Gluckman, 2011 ; Bjorklund, 2006 ; Harper, 2005 ). In particular, there is growing appreciation of the role of epigenetic changes in disease generation, including cancer, cardiovascular disease, and type 2 diabetes ( Gluckman & Hanson, 2004 ). In this light, Gluckman, Hanson, and Beedle (2007) have recently suggested that the obesity epidemic in the United States and elsewhere is likely affected by dietary events that occurred in the parental generation and possibly in earlier generations as well.

The Framework of Relational Causality

The emerging findings from epigenetic research indicate that exposure to specific nutrients, social stimulation, or other types of environmental factors can influence gene expression without altering gene sequences, and further, such influences can persist through adulthood and in some cases be transmitted to subsequent generations. In other words, a wide range of nongenetic and environmental factors are now recognized as key participants in gene activity and expression, in some cases well beyond the timescale of individual development. This expanded perspective on the nature of genetic activity and inheritance is resulting in a new way of thinking about biological causality, which has been termed the relational concept of causality ( Gottlieb & Halpern, 2002 ). From this view, the causes and control for development do not reside in any one factor or component but rather in the nature and dynamics of the relations between factors internal and external to the organism. Kaplan (2006 , p. 50) has summarized this key insight as follows:

every trait of an organism is the result of the interaction of various genes and environments during the developmental process. In order to be successful, organismal development always requires the presence and coordinated actions of various kinds of resources (genetic, epigenetic, and environmental, to name a few), so it makes no sense to ask if a particular trait is genetic or environmental in origin. Understanding how a trait develops is not a matter of finding out whether a particular gene or a particular environment causes the trait; rather it is a matter of understanding how the various resources available in the production of the trait interact over time.

This relational point of view shifts thinking about development away from the internally based, prespecified outlook prevalent for most of the last century and toward an appreciation of development as a situated process dependent on resources distributed across the organism–environment system. In other words, control of development is not prescribed by the genes; rather, control is exerted by the regulatory dynamics of the gene-in-a-cell-in-an-organism-in-an-environment system ( Mahner & Bunge, 1997 ; Oyama, Griffiths, & Gray, 2001 ; Robert, 2004 ). Given that development is always the result of a series of elaborate temporal and spatial interactions within and between levels that are inherently context-dependent (e.g., Coen, 1999 ; Goodwin, 1994 ; Nijhout, 1990 ; Noble, 2006 ), it is not possible to meaningfully assign “control” to any one variable of the developmental system.

This distributed view of developmental causation does not suggest that genes do not play a necessary and significant role in the developmental process, nor does it argue against heritable changes in the phenotype originating in the genotype. However, it has become clear that the passing on of genes from one generation to the next is not a sufficient explanation for the achievement of phenotypic outcome or for evolutionary change. What are passed on from one generation to the next are genes and a host of other necessary developmental resources that contribute to the realization of an individual’s traits ( Griffiths & Gray, 2004 ; Jablonka & Lamb, 1995 ; Oyama, 1985 ). These “cycles of contingency” ( Oyama, Griffiths, & Gray, 2001 ) highlight the fundamental importance of experience and context and also point to the dividends of expanding our framework for developmental inquiry beyond the lifecycle of the individual.

The developmentalist Susan Oyama (1989) outlined this expanded “systems” perspective on the transmission of developmental resources between generations, including (a) the genes, (b) the cellular machinery necessary for their functioning, (c) the extracellular environment, and (d) the larger context, which can include the maternal reproductive system, parental care, or interactions with other conspecifics, as well as other aspects of the animate and inanimate world. This systems perspective moves us away from characterizing genes as sources of plans, instructions, or information and toward a focus on the molecular and cellular events in which genes actually operate during development (see Johnston & Edwards, 2002 , for a useful illustration). A developmental systems approach also raises significant challenges to the established gene-centered view of evolution that gave rise to sociobiology, behavioral ecology, and evolutionary psychology in the last half of the twentieth century ( Lickliter & Honeycutt, 2003 ).

Exploring the Links Between Development and Evolution

A longstanding problem of evolutionary theory has been how to account for the sources of phenotypic stability and phenotypic variability observed within and across generations. As we have seen, genes were thought to be the answer to this problem for most of the last century. However, over the past several decades a different account of phenotypic stability and variability has taken shape. This account is based on a relatively simple but nonetheless key insight: given that all phenotypes arise during ontogeny as products of individual development, it follows that a primary basis for phenotypic stability and variability must be the patterns and processes of development . As the morphologist Pere Alberch (1982) pointed out several decades ago, development contributes to the evolutionary process in at least two primary ways. First, it generates the reliable reproduction of phenotypes across generations and constrains phenotypic diversity by limiting the “range of the possible” in terms of both form and function. This “robustness” of development, despite genetic or environmental perturbations, is the regulatory function of development ( Maynard-Smith et al., 1985 ; Siegal & Berman, 2002 ; Wimsatt, 1986 ). Second, development introduces phenotypic variation and novelties of potential evolutionary significance. This is the generative function of development and provides a key source of variation upon which natural selection can act ( Gottlieb, 2002 ; Johnston & Gottlieb, 1990 ; West-Eberhard, 2003 ). The regulative and generative aspects of development indicate that the natural selection of random genetic mutations (the cornerstone of the Modern Synthesis framework) cannot be sufficient to account for evolution. A growing acknowledgement of this insight over the past several decades has fostered a renewed interest in development within evolutionary biology and increasing recognition that changes in evolution reflect changes in developmental processes (e.g., Arthur, 2002 ; Gilbert, Opitz, & Raff, 1996 ; Gottlieb, 1992 ; Pigliucci, 2007 ; Raff, 1996 ).

This critical reassessment of the links between development and evolution has also contributed to the coalescence of one of the most rapidly growing fields within contemporary biology, evolutionary developmental biology. Evolutionary developmental biology (usually referred to as evo-devo ) involves a partnership among evolutionary, developmental, and molecular biologists to integrate our understanding of developmental processes operating during ontogeny with those operating across generations (e.g., Arthur, 1997 ; Hall, 1999 , Kirschner & Gerhart, 2005 ; Love, 2003 ; Raff, 2000 ). Unlike the geno-reductionistic premises of the Modern Synthesis, evo-devo views evolution as changes in developmental processes rather than simply changes in gene frequencies. This agenda addresses a variety of concerns, including how modifications in developmental processes lead to the production of novel phenotypes, the role of developmental plasticity in evolution, and how ecology influences developmental and evolutionary change ( Hall & Olson, 2003 ).

Of particular importance to this approach is the fact that phenotypic variability and novelty (i.e., plasticity) can be generated by both genetic and nongenetic means. Phenotypic plasticity can be defined as “the ability of an organism to react to an internal or external environmental input with a change in form, state, movement, or rate of activity” ( West-Eberhard, 2003 p. 33). This capacity for phenotypic plasticity was long considered to be genetically determined by most biologists (e.g., Mayr, 1942 ; Via & Lande, 1985 ). However, the rich interplay between genes and their environments demonstrated by contemporary epigenetic research has suggested a range of mechanisms whereby developing individuals can modify their morphology, physiology, or behavior in response to the specific features of their context or habitat. For example, the timing of hormone production and the sensitivity of organs and tissues to the presence of hormones can be readily altered by features of the environment, and both can result in significant changes in morphology and behavior (e.g., Nijhout, 1999 ; Schlichting, 2004 ). This is the case because an organism’s nervous system can ongoingly monitor its environment and rapidly change the hormonal milieu within the organism. Hormones in turn alter gene expression patterns, modify metabolic rates in target cells, and ultimately mediate behavioral and morphological changes in the phenotype ( Gilbert, 2005 ).

Recent research with desert locusts has provided a striking example of such a developmental cascade ( Anstey et al. 2009 ). The locust is usually cryptic in color (green) and solitary, avoiding other locusts and flying alone at nighttime. However, under favorable climatic conditions that result in an increase in vegetation, the numbers of these solitary locusts can explode, triggering a rapid increase in population density and a rapid transformation in their color (now bright yellow) and social behavior. Typically solitary locusts now form bands and eventually form swarms consisting of billions of locusts. This rapid transformation is known to include many morphological, physiological, and behavioral features and to involve numerous chemical messengers and the expression of more than 500 genes ( Kang et al. 2004 ). Anstey and colleagues (2009) have shown that a key agent in this remarkable phenotypic plasticity is the neurotransmitter serotonin, which is synthesized in the locust’s thoracic nervous system in response to the multiple sensory cues (touch, smell, or sight) provided by social contact with other locusts when population density increases. Within as little as 2 hours of proximity to other locusts, elevated serotonin levels switches behavior from mutual aversion to mutual attraction, recruiting additional hormones and chemical messengers and allowing the formation of the enormous locust swarms that can wreak havoc on human populations. Remarkably, serotonin-containing neurons in locusts comprise only five cell pairs in each thoracic compartment of their nervous system ( Tyrer, Turner, & Altman, 1984 ). Serotonin has also been implicated in changing aggressive and courtship behavior after social interactions in a number of species, including other insects ( Hofman & Stevenson, 2000 ), crustaceans ( Kravitz & Huber, 2003 ), and mammals ( Miczek et al. 2007 ).

The growing recognition across the biological sciences of the need to consider and define the complex interactions among genetics, development, and ecology in order to understand the range of morphological structures, shifts in behavioral repertoires, and other instances of phenotypic plasticity observed across plant and animal species (e.g., Gilbert, 2001 ; Nijhout, 2003 ; Pigliucci, 2001 ; Schlichting & Pigliucci, 1998 ; West-Eberhard, 2003 ) is expanding the scope of experimental inquiry. This new approach to phenotypic plasticity views the novelty-generating aspects of evolution as being the result of the developmental dynamics of living organisms, situated and competing in specific ecological contexts, and not simply the result of random genetic mutations, genetic drift, or recombination.

Johnston and Gottlieb (1990) have provided a useful illustration of this view of phenotypic plasticity with an example of a population of rodents whose normal diet consists of soft vegetation such as fruit and leaves. In their scenario, climate changes in the local environment result in the rodents encountering a new food source of relatively hard but highly nutritious seeds. As some of the animals increase the representation of seeds in their diet, a number of developmental effects of their new diet become evident, both in themselves and in some cases in their offspring, including changes in body size, age of sexual maturation, and indirect changes in their morphology. For example, as the diet changes from soft vegetation to harder seeds, the mechanical stresses exerted on the growing jaw tissues during development change. Given that patterns of bone growth are known to be determined in part by the nature of the forces exerted on the growing bone ( Bouvier & Hylander, 1984 ; Frost, 1973 ), the skeletal anatomy of the jaw and teeth will be different in the animals that experience the hard seed diet early in life. Thus, behavioral change in members of a population (a preference for a new diet of hard seeds) can lead to specific anatomical changes (modification of the jaw and teeth), and those changes can endure across generations, so long as the new diet remains available. Johnston and Gottlieb (1990) refer to this process as neophenogenesis , a term first proposed by the behavioral embryologist Zing-Yang Kuo (1967) to refer the emergence of novel phenotypes as a result of modification in species-typical experience during development.

Recent research with Darwin’s finches, famous for their role in Darwin’s formulation of the principle of natural selection, has provided an elegant example of how the complex interplay of molecular, cellular, and ecological factors can contribute to relatively rapid and dramatic phenotypic change (in this case, the variety of beak shapes observed across these 13 species of finches distributed across the Galapagos Islands). Such developmental plasticity provides a potent pathway for organisms to rapidly change structure and function in response to environmental resources and changes (see West-Eberhard, 2003 , for additional examples). Evidence suggests there is a substantial amount of hidden variation in the genome of most organisms ( Schlichting & Pigliucci, 1998 ); this plasticity potential and its implications for development and evolution have rarely been studied systematically and provide a rich field for future experimental analysis.

In the case of Darwin’s finches, in the time frame of just 1 to 2 million years, a founding group of finches from South America generated more than a dozen different finch species on the remote Galapagos Islands, including some with large, pliers-like beaks capable of cracking nuts and seeds and some with forceps-like beaks able to extract insects from fruit. Darwin had noted these birds’ remarkable differences in beak size and shape on his visit to the Galapagos Islands during his Beagle voyage in 1835, but due to the degree of variation across species did not realize at the time that they were all finches. Further reflection on this variation after his return to England contributed to Darwin’s formulation of the critical role of natural selection in the direction of evolutionary change.

The standard genocentric explanation of the striking variation in beak size and shape seen across these closely related finch species proposes that genetic mutation, recombination, and reassortment of genes in an island’s founder population would occasionally result in variant birds that had somewhat smaller and more forceps-like beaks or somewhat larger and more pliers-like beaks than those of the founder population. These individuals would be more likely to explore and exploit different food niches (insects vs. seeds), potentially leading to increasing geographical and behavioral isolation from one another. Morphological change would be gradual in this scenario, but over many generations differential reproduction (based in part on relative feeding success) would eventually result in the selection of several variations of the founders’ beak type.

Recent synthesis of molecular, cellular, and ecological research indicates that the pathway to the remarkable variations observed in beak size and shape is more contingent and more rapid than traditional views of evolutionary change would suggest (e.g., Abzhanov et al., 2004 , 2006 ; Grant, Grant, & Abzhanov, 2006 ). Current evidence indicates that the size and shape of the finch beak are determined during development by the growth and differentiation of neural crest cells that settle around the mouth of the developing bird embryo. These neural crest cells produce a growth factor protein called bone morphogenetic protein 4 (Bmp4), which stimulates the deposition of bone and beak materials during embryogenesis. This protein is produced earlier in embryonic development and at higher levels in the finch species with larger and wider beaks than in the closely related finch species with longer and narrower beaks ( Abzhanov et al., 2004 ). Interestingly, when Bmp4 is experimentally introduced into the beak neural crest cells of chicken embryos, they also develop broader and larger beaks than control chicks. The introduction of other growth factors did not have this effect. Related work has found that a protein that mediates calcium signaling and plays a role in cell and tissue differentiation (calmodulin or CaM) is expressed at higher levels in finch species with longer, narrower beaks than in those with longer, wider beaks ( Abzhanov et al., 2006 ). It appears that a variety of interrelated factors, including the number of neural crest cells, the level of signaling that stimulates or inhibits the production of growth factor protein and calmodulin, and the types of signals that induce cell death of the neural crest cells, are all at play in generating the beak shape variation seen across these finch species. How these various factors and their relations are regulated by the birds’ experience and ecology (particularly the type of food sources available) are not fully understood, but given the wide adaptability of neural crest cells, it seems that relatively large modifications in beak size and shape have been accomplished with relatively few changes in the developmental process. This potential for rapid phenotypic adjustment has important implications for evolutionary change, in that it would increase the likelihood that members of the population could quickly take advantage of new or changing resources and habitats ( Gottlieb, 2002 ).

Examples such as these have led some developmental and evolutionary biologists to propose the notion of evolutionary capacitance , the idea that accumulation of hidden genetic variation and developmental potential can come into play when developing organisms are challenged by novel or unusual developmental conditions ( Gottlieb, 2002 ; Masel, 2005 ; Rutherford & Lindquist, 1998 ). Perhaps the most remarkable example of this pathway to phenotypic novelty, whereby an organism’s new experiences or conditions can activate previously inactive genes and developmental potential, is the demonstration that chickens can be induced to grow teeth ( Kollar & Fisher, 1980 ). Under typical prenatal conditions, when the chick embryo’s oral epidermis and oral mesenchyme cells interact, the embryo grows the usual, species-typical chick beak. However, when the chick embryo’s oral epidermis is placed in contact with mammalian (mouse) molar mesenchyme during embryogenesis, the embryo produces enameled dentition (a mammalian tooth) rather than a chick beak, demonstrating the hidden developmental-genetic potential available for the generation of phenotypic novelty or variation.

Because of the variability of relevant resources across different environments and because only a portion of the genome is expressed in any individual (due to its specific developmental context and experience), what is actually realized during individual development represents only one of many possibilities. This insight is a core tenet of probabilistic epigenesis, the view of development that emphasizes that because of the multiplicity of levels, factors, and interactions involved and because of its history-dependent and situated nature, neither physical nor behavioral development can have a predetermined trajectory ( Gottlieb, 2007 ). To understand the origin, maintenance, or transformation of any phenotypic trait, it is necessary to study its development in the individual. This has significant consequences for how we approach making sense of phenotypic stability and variability. Contrary to the common assumption still evident in some quarters of developmental psychology that phenotypic stability is “biologically” based and phenotypic variability is “experience” based, there are not separate or distinct processes responsible for stability on the one hand and variability on the other. The same developmental processes that regulate stability also regulate variation . Both are the products of the bidirectional traffic among the various networks, resources, and levels of the organism–environment system (see Sholtis & Weiss, 2005 , for further discussion).

We are just beginning to have the conceptual and technological tools to build this “systems-level” understanding of development. However, initial efforts have been under way in the study of biological development for years. For example, Atchley and Newman (1989) explored several types of factors—genetic, maternal, environmental—that affect the stability and variability of developmental outcomes. Their model for integrating genetics with developmental analysis recognizes that multiple factors, including contingencies in mating (which create the developing organism’s genome) and contingencies of the maternal environment in which the individual develops (including cytoplasmic and uterine factors), mediate the variability and stability of phenotypic outcome. In other words, factors operating at the level of the genes and factors operating at the level of the organism mutually influence one another, and both levels of influence are always involved in the development and maintenance of morphological and behavioral phenotypes ( Gottlieb, Wahlsten, & Lickliter, 2006 ; Moore, 2008 ).

Conclusions: Toward an Integrative Developmental Science

Most biologists and psychologists have come to appreciate that physical, behavioral, and cognitive development is far more complex and dynamic than was assumed from the genocentric view of the last century. In addition to genes, individuals inherit an entire developmental system, including cell assemblies, an embryonic and fetal stimulative environment, as well as parents, peers, and the places they inhabit ( Lickliter, 2005 ; Oyama, Griffiths, & Gray, 2001 ; West & King, 1987 ). Current evidence indicates that gene–environment relations occur at all of these levels. The process of development is thus inherently historical and situated and the causes of developmental outcomes are to be found in the dynamic relations among the complex array of internal and external resources occurring across the organism–environment system.

Biology’s “Modern Synthesis” of the twentieth century entailed a false split between internal and external factors contributing to development, thereby reifying and perpetuating the nature–nurture controversy. As Overton (2006 , p. 43) has pointed out: “the controversy is supported by the neo-Darwinian radical rupture of the whole into an inside (gene, biology) story that comes to be called nature, and an outside (social-cultural, experience) story called nurture…the controversy becomes the questions of which one fundamentally determines change, or how much does each contribute independently to determining change.” As most readers are well aware, numerous methods have been devised and promoted to separate the effects of genes and environment both experimentally and statistically (see Moore, this volume 1).

Despite persuasive efforts within the psychological sciences to integrate conceptions of nature and nurture over the past several decades (i.e., Gottlieb, 1997 ; Lerner, 2006 ; Moore, 2002 ; Overton, 2006 ; Oyama, 1985 ; Richardson, 1998 ), developmental psychology has continued to struggle with versions of the nature–nurture debate. Within the domains of perceptual and cognitive development, this struggle has often centered around the issue of the extent to which humans are innately prepared to interpret and act on the world and the extent to which they rely on learning and experience (see Blumberg, 2005 , and Stiles, 2008 , for discussions). For example, Spelke and Newport (1998 , p. 323) have argued for the differential roles of biology and experience, suggesting that a solution to the nature–nurture debate is the “thesis that human knowledge is rooted partly in biology and partly in experience and…that successful explanations of the development of knowledge will come from attempts to tease these influences apart.” This dichotomous view is contradicted by the findings from epigenetics reviewed earlier in this chapter. If there is any lesson to be learned by developmental psychologists from recent advances in the life sciences, it is that “biology” and “experience” are completely intertwined and cannot be meaningfully separated or “teased apart” ( Keller, 2011 ).

For example, as discussed in preceding sections, experiential factors such as day length or social interactions alter the release of hormones, and these hormones are capable of diffusing into cell nuclei and binding with DNA, thereby regulating the activation of DNA transcription, “turning on” specific genes and their specific protein products ( Cheng, 1979 ; Yamamoto, 1985 ). Similarly, experiments on the early development of the nervous system show that the amount of protein synthesis going on can be regulated by neural activity (e.g., Born & Rubel, 1988 ), again illustrating the interdependent coaction of genetic and experiential influences at play during individual development. Decades of studies of brain development in animals and humans indicate that the structure and the function of the nervous system is always the product of dynamic processes involving interactions that extend from the genes to the environment (see Stiles, 2000 , 2008 ), again emphasizing the historical, situated, and experience-dependent nature of phenotypic development.

As pointed out by several developmental psychobiologists (e.g., Gottlieb, 1997 ; Kuo, 1967 , Schneirla, 1956 ) over the past half-century, it is important to remember that experience is not synonymous with learning but rather refers much more broadly to function or activity, including the electrical activity of neurons, neurochemical and hormonal secretion, the use of muscles and sensory systems, and the behavior of the organism itself. Attempts to delineate between the relative causal power of internal versus external factors or between the “biology” and “experience” thought to be associated with any given behavioral or cognitive ability are thus unnecessarily reductionistic and are not supported by our current understanding of the dynamics of biological or psychological development. Further, attempts to dichotomize developmental causality effectively serve to “black box” the multiple levels of influence, resources, and interactions between gene and behavior, thereby isolating developmental psychology and its concerns from developmental genetics, developmental biology, and developmental neuroscience ( Gottlieb, Wahlsten, & Lickliter, 2006 ).

A more integrative approach to addressing the complexities of the dynamics of development will involve bringing together genetics, epigenetics, molecular, cellular, and developmental biology, neuroscience, developmental psychology and psychobiology, and evolutionary biology to construct a more comprehensive explanation of the ways and means of the stability and variability of phenotypic development (e.g., Lickliter & Honeycutt, 2003 ; Müller, 2007 ; Müller & Newman, 2003 ; Neumann-Held & Rehmann-Sutter 2006 ; Overton, 2006 ). Exactly how this synthesis will ultimately play out is not yet clear, but it is increasingly apparent that biological or psychological theories of development do not make sense outside the perspective of the organism–environment system.

Future Directions

The new directions in thinking about causality being taken in developmental science are providing scientists with novel problems, creating new lines of research, and forging links between what have been seemingly unrelated areas of investigation. As developmental science has matured over the past 25 years, several key insights regarding how to think about the relations among development, heredity, and evolution have taken shape that will likely steer directions in theory and research in the years ahead. These insights include the following:

More sophisticated models of relational causality are necessary to both inform and advance developmental and evolutionary inquiry (see Salthe, 1985 ). This will include leaving behind the idea that developmental causation exists at the level of the genes, a point of view that discounted the organism, its actions and experience, and the features of its physical, biological, and social environment relevant to the developmental process ( Gilbert, 2005 ; Reid, 2007 ).

Gene expression is a dynamic and contingent process and regularly involves factors external to DNA. These include multiple factors within the environment of the cell as well as multiple factors external to the cell, many occurring beyond the boundary of the organism. This distributed and contingent regulatory network dismisses the notion that one can meaningfully separate or partition genetic (internal) and environmental (external) influences on human behavior and development. The effort to synthesize molecular, cellular, developmental, and evolutionary biology that is under way (see Crews, 2008 ; Jablonka, 2007 ; van Speybroeck, 2000 ) will undoubtedly provide more comprehensive and cohesive models of phenotypic development.

The emergent properties of phenotypic stability and phenotypic variability arise from an individual’s entire developmental system ( Lickliter & Harshaw, 2010 ). This insight points to the importance of research explicitly focused on the patterns and nature of the relations and interactions among elements and levels of the developmental system. This effort will require cross-level frameworks that investigate the linking and modeling of interactive processes occurring at different levels of analysis. Recent advances in “systems biology” (e.g., Bruggeman & Westerhoff, 2007 ; Kitano, 2002 ; O’Malley & Dupré, 2005 ) are providing important technical, methodological, and conceptual steps in how to proceed in this direction. In particular, systems biology is advancing our abilities to address the underlying issues involved in identifying systems and to model how causality can operate across different levels of organization.

Early experience shapes how individuals will respond to later experience; later experience in turn modifies the effects of these earlier experiences (see Crews, Lou, Fleming, & Ogama, 2006 , for examples). Recent advances in the neurosciences demonstrate the interplay of genes, cells, neurotransmitters, and hormones, as well as the nature and patterns of social interaction on the course of plasticity and learning across the lifespan. These findings emphasize the historical nature of development and highlight the importance of research efforts focused on a fuller understanding of the particulars of the prenatal and perinatal periods and their contributions to our understanding of lifespan development (e.g., Bateson & Gluckman, 2011 ; Gluckman & Hanson, 2005 ).

Comparative research has revealed pathways leading from the behavior of the mother to long-term modification of gene expression and behavior in offspring ( Szyf, Weaver, & Meaney, 2007 ). Additional research is needed to determine whether similar mechanisms are at play in generating interindividual differences in human behavior. Available evidence from primates indicates that early influences can play a key role in the lifetime risk of disease and other adverse outcomes ( Bennett, 2008 ), suggesting the likely importance of such pathways to our understanding of human development.

As in any science, the models that formed the foundation of our current knowledge of development have to be reevaluated and updated as new evidence is obtained. Several of the themes and theories used to address biological development during the nineteenth and twentieth centuries are proving to need such revision. As I have reviewed in this chapter, established assumptions regarding the role of genes in development, heredity, and evolution ( predetermined epigenesis , heredity as gene transmission , and genetic encapsulation ) are being seriously challenged and rejected by demonstrations of the environmental regulation of gene expression and cellular function, as well as the effects of sensory stimulation and social interaction on neural and hormonal responsiveness. The epigenetic revolution under way in the biological sciences represents a significant reorientation in how we attribute cause in the study of development. It is also deepening our appreciation of the complex array of developmental resources, hidden regulators, and experiential nuances at play in the process of human development. A great deal remains to be discovered.

Five Questions for Future Research

Cross-level frameworks that investigate the linking and modeling of interactive processes occurring at different levels of analysis are needed to advance our understanding of the intricate dynamics of development. How can developmental psychology form effective interdisciplinary alliances with genetics, epigenetics, cellular and developmental biology, and neuroscience?

Comparative research has revealed pathways leading from the behavior of the mother to long-term modification of gene expression and behavior in her offspring. Systematic research is needed to determine whether similar mechanisms are at play in generating interindividual differences in human behavior. What methods and analyses will best allow developmental psychology to identify and assess the transgenerational effects of individual experience?

Developmental plasticity provides a potent pathway for organisms to rapidly change structure and function in response to environmental changes. Evidence suggests there is a substantial amount of hidden variation in the genome of most organisms. This plasticity potential and its implications for development and evolution have rarely been studied systematically and provide a rich area for future experimental analysis.

Decades of studies of brain development in animals and humans indicate that the structure and the function of the nervous system are the product of dynamic processes involving interactions that extend from the genes to the environment. How does individual experience, including the electrical activity of neurons, neurochemical and hormonal secretion, the use of muscles and sensory systems, and the behavior of the organism, guide and constrain the development and modification of the nervous system across the lifespan?

The recent reassessment of the links between development and evolution in biology has raised a number of new research questions, including how modifications in developmental processes lead to the production of novel phenotypes, the role of developmental plasticity in evolution, and how ecology can influence developmental and evolutionary change. How can developmental psychology contribute to current efforts to answer these challenging questions?

Acknowledgments

The writing of this chapter was supported by NICHD grant RO1 HD048423 and NSF grant BCS 1057898. I thank Lorraine Bahrick, Eric Charles, and Chris Harshaw for their constructive comments.

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Biological Factors That Influence Child Development

Early child development  is influenced by a wide variety of both biological and environmental factors . Biological factors can play a particularly important role in early development. These factors influence a child in both positive and negative ways. They can affect children throughout their development, particularly during critical times such as the prenatal period and early childhood.

Research conducted at Rutgers University demonstrated that prenatal factors affect linguistic development and postnatal factors contribute to a child’s cognitive development.   And gross motor development is widely considered to be the result of innate, biological factors, with postnatal factors contributing to a lesser extent. 

Biological factors include genetic influences, brain chemistry, hormone levels, nutrition, and gender. Here is a closer look at nutrition and gender and how they affect development.

Proper nutrition is a vital factor in a child’s overall development. Prior to birth, a mother’s diet and health play a key role. For example, folic acid intake of 400 micrograms (mcg) daily for three months prior to conception and during early pregnancy significantly decreases the risk of certain birth defects of a baby’s brain (anencephaly) and spine (spina bifida).

These birth defects occur in the first few weeks of pregnancy, which is why it is important for women in their childbearing years to ensure they are getting at least 400 micrograms of folic acid daily. Waiting until a woman finds out she is pregnant can be too late.

Most people possess 23 pairs of chromosomes in their cells (with the exception of special reproductive cells called gametes). The first 22 pairs are called autosomes, which are the same in boys and girls. Therefore, males and females share most of the same set of genes.

The 23rd pair of chromosomes is what determines the gender of an individual. Boys typically have one X chromosome and one Y chromosome while girls have two X chromosomes. Hence, gender differences at the biological level are found on the Y chromosome. Gender can influence development in a variety of ways. For example, boys tend to develop and learn differently than girls and have lower levels of school readiness.  

A child’s physical body has distinctive reproductive organs and becomes further differentiated as special sex hormones are produced that play a role in gender differences. Boys typically produce more androgens (male sex hormones), while females produce estrogens (female sex hormones).

Scientists have studied the effect of excessive amounts of sex hormones on a child’s behavior. They have found that boys with higher than normal androgen levels play and behave similarly to their male peers with normal androgen levels. However, girls with high androgen levels typically exhibit more gender-stereotypic male traits than do girls who have normal androgen levels.  

Interactions Among Biological and Environmental Factors

It is important to remember that biological factors do not act in isolation. Genes, for example, can interact both with other genes and the environment. Some genes may dominate and prevent others from being expressed. In other cases, certain biological influences might impact genetic expression.

An example of biological influence over gene expression is a child not getting proper nutrition. The child might not grow tall, even though they have inherited genes for height.

In order to understand child development, it is essential to consider all the many factors that may play a role. Healthy development is not the result of a single influence.

A Word From Verywell

The first three years of a child’s life is a period of tremendous growth and development. It is characterized by rapid development, particularly of the brain, where connections between brain cells are being made to provide the necessary building blocks for future development. For children to learn and become resourceful and independent, it is important to devote attention to early childhood development.

Stromswold K. Why aren't identical twins linguistically identical? Genetic, prenatal and postnatal factors . Cognition . 2006;101(2):333-84. doi:10.1016/j.cognition.2006.04.007

Centers for Disease Control and Prevention. Folic acid.

Janus M, Duku E. The school entry gap: Socioeconomic, family, and health factors associated with children's school readiness to learn .  Early Educ Dev. 2007;18(3):375-403. doi:10.1080/10409280701610796a

Berenbaum SA, Beltz AM. Sexual differentiation of human behavior: effects of prenatal and pubertal organizational hormones . Front Neuroendocrinol. 2011;32(2):183-200. doi:10.1016/j.yfrne.2011.03.00

Centers for Disease Control and Prevention. Folic acid helps prevent some birth defects .

Irwin LG, Siddiqi A, Hertzman C. Early childhood development: A powerful equalizer . World Health Organization.

By Douglas Haddad Douglas Haddad is an award-winning teacher and best-selling author, covering learning disabilities and other topics related to education.

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Biological Psychology: Development and Theories Essay

1. introduction.

The first section of this essay provides a brief introduction to the nature of biological psychology and its significance in the field of psychology as a whole. The author emphasizes the value of this knowledge to the student of general psychology and provides some insight into the range of applications for this information, stressing its relevance to other areas of psychology and to other fields in the sciences. Finally, the reader is given a preview of the insights to come through an outline of the subsequent chapter content. In 1.1, the fundamental tenets of biological psychology are addressed through an elucidation of the crossover between cognitive psychology and neuroscience. By examining the ways in which the brain and the nervous system are capable of influencing, and being influenced by behavior and mental processes, the reader is given a sound understanding of why biological psychology is important to the broader field of psychology. This is further elucidated through a breaking down of 'what is meant by a biological approach', in terms of both the levels of explanation in psychology, and the basic assumption that all that is psychological is first physiological. This section effectively provides a platform upon which to explore the following chapter content. Section 1.1 concludes with an outline of the history of the biological approach, demonstrating to the reader that the content to come is not purely theoretical, but that throughout the ages, research and application of the biological approach has proven to be highly influential to behavioral science.

1.1. Definition of Biological Psychology

Biological psychology is a branch of biology which attempts to consider causes of controversies of mental and emotional dispositions. It is an interdisciplinary approach to the study of behaviour, thought and emotion; weakness and abnormalities as well as patterns of living and living. The ultimate goal of this work is to deal with the relationship between the brain and abnormal behaviour. However, it is important to know that many aspects of behaviour are difficult to measure, so biological psychology is faced with many methodological problems. These stem from the issues of defining loosely held physiological diseases in comparison to clear-cut anatomical diseases. This is an important factor because a firm definition of disease can lead to adequate treatment and possible prevention in the future. There is also a problem when we consider animal models. An animal model is an attempt to find a simpler system that can be studied; one that has basic mechanisms that contribute to a pathological state in humans. These studies have been successful in understanding a variety of problems, ranging from phenomena such as learning and memory to using addictive substances. However, we must bear in mind that because these are simpler systems, the behaviour and constructs may be different from human models. In addition, the use of invasive techniques, including brain lesions and administration of drugs that may have devastating or long-lasting effects, are commonly used to see how the brain regulates normal and abnormal behaviour. The problem with this is weighing the benefits of the knowledge to be acquired against any untold suffering involved, as well as addressing any moral or ethical issues with regards to using animals as subjects, especially where the decision to end their lives may be necessary.

1.2. Importance of Studying Biological Psychology

Understanding the brain's interaction in human behaviors is a complex undertaking, involving several methods of enquiry and a number of disciplines. Neuroscientists examine the brain in many ways. Biological psychologists often use animals as subjects because their behavior can be more easily manipulated than that of humans. Such experiments with animals provide a more precise understanding of how the brain functions because the scientists can control the variables in the experiments. But knowledge about the brain and its relation to the mind is also being uncovered through research in cognitive neuroscience at the molecular and cellular levels. This is sometimes done with human subjects in studies that examine the effects of brain injuries, the effects of drugs, or the genetics of certain psychological disorders. Still, other insights into the brain and behavior have come from clinical observations and discoveries made in the treatment of mental illness. Each of these approaches to studying the brain and behavior has its own techniques, its own level of explanation, and its own value. On a scientific level, we can see that understanding how the brain affects human behavior can be very valuable. Take, for example, psychologists who practice cognitive theory and aim to help people change negative thinking patterns into thoughts that are more positive. Cognitive theory states that emotions and behaviors are caused by internal events. By this, they mean that what we feel and what we do is produced by what we think. Here's where the brain comes in: Cognitive psychologists believe that changes in thinking can sometimes be brought about by biological changes. For example, a depressed person who feels there is no hope may challenge his assumption that an event is hopeless by tackling it. He may then feel less depressed and more hopeful. The behavior is the tackling of the event. The change in feeling results from, and is caused by a change in thinking, what the person tells himself has changed, and what could be more biological than changes in brain activity? Through the use of biological psychology, we can further understand the relationship between changing thinking patterns and brain activity, and thus better understand how cognitive theory can best be put into practice.

2. Development of Biological Psychology

Biological psychology can be traced back to the time of the early Greeks. The link between the brain and the mind has been the foremost topic in the history of biological psychology. This debate of where the mind exists has been in existence for centuries, going back to the time of the early philosophers. This topic was further explored during the Renaissance period by the work of the French philosopher René Descartes. He proposed that the body and mind were two separate entities and described bodily functions as reflexes controlled by the mind. He believed that the mind controlled the movements of the body through the pineal gland. This idea of dualism, where the mind and body were separate entities, continued to dominate the history of biological psychology. The concept of physiological psychology was developed in Germany in the 18th century, some 2000 years after the time of Hippocrates. This is the study of the connections between environmental and physiological processes of the body, and how it affected the mind. This came to pass in the early to mid-19th century when localization of function was a major topic in the discussion of physiological psychology. This idea, that various parts of the brain perform different tasks, was deduced by studying the effects of accidental injury to specific parts of the brain. This led to further research into biological psychology and to the various fields of neuroscience today. The following sections focus on the milestones and the key figures that have helped to shape the vast and expanding field of biological psychology. These significant changes in ideas and events have led to its current standing as a valid and important field in psychology today.

2.1. Historical Overview

The term biological psychology was first used in the late 1800s. James Mark Baldwin introduced the terminology to describe the mental development of animals and humans. It was not until the 1960s that the term biological psychology was adopted. The field of neuroscience is rooted in medical science. For this reason, it is important to look at the beginning of brain investigation and the discoveries that led us to where we are today. It was the Egyptians who first identified the brain as the control center for the body. This idea was further explored by Hippocrates, who believed that the brain was the seat for intelligence. The father of modern medicine, Hippocrates, is also the father of cerebral localization. He believed that the brain was the most important organ for intelligence and that it was the seat for all sensory experiences. In the 2nd century AD, the Alexandrian Greek physician, surgeon, and philosopher Galen made significant contributions to understanding the relationship between the brain and the nervous system, which he believed to be the seat of the mind. He also wrote about various brain disorders. During the Dark Ages and the Middle Ages, brain research was more philosophy than practice, and few findings were made.

2.2. Key Figures in the Field

This section examines persons who have had a significant impact on the field of biological psychology. Obviously, the study of the physiology of behaviour and the functioning of the nervous system can be traced back quite far in history, and often individuals who were not specifically in the field of psychology had made contributions to it. This section will only consider persons who have had a direct influence in the evolution of biological psychology and whose specific interests were in the workings of the brain and the nervous system as related to behaviour. This selection is necessarily limited, and it may be surprising that some persons who were quite historically important have been left out. J.B. Watson, the founder of behaviourism, was known to hold disdain for the field of physiological psychology, and although his work posed quite a challenge to biological psychology, it primarily served to hinder the advancement of the field until after his death. Similarly, due to the decline of biological psychology in the 1950s and 60s, many famous psychologists of that time still had little to no influence in the field.

2.3. Milestones in the Development of Biological Psychology

Milestones in the development of biological psychology start with the identification of various activities carried out by small numbers of people over a long period of time that have created this field today. Many researchers and clinicians have made significant advancements, but five events are particularly noteworthy. First, the formative event was the meeting of the American Psychological Association that decided to make an attempt to create a biological research society, leading to the creation of 'Psychophysiology'. The second event is the redefining of the orientation of clinical problems into scientific research, a phase that is termed as Psychiatric Research in Washington DC. This resolve to reorient resulted in a number of accomplishments that include a better understanding of mental diseases, developing methods to study these diseases, and ultimately, developing a new field of neuroscience. One example of these achievements is a study that predicted the response of syphilis-induced cerebral rot. This thematic program, led by the directorship of Frederic A. Gibbs, prominently created the first contemporary association between psychology and psychiatry at the Research Information Center. The third event is the completion of the first academic training program that taught career researchers biological studies that are relevant to psychological problems. This event leads to the fourth, which is the formation of the Biological Psychiatry Data Center, and finally, the creation of the field of Biological Psychiatry.

3. Theories in Biological Psychology

Evolutionary psychology seeks to reconstruct problems that our current species faced in its ancestral environment and the problem-solving behaviors that resulted to allow for gene replication to occur. This functional way of thinking about biological mechanisms and behaviors focuses on the utility and purpose of the psychological traits seen in modern people. The theoretical foundations of evolutionary psychology are the general and guiding properties of natural and sexual selection. Some psychologists believe that evolutionary psychology is a new way of thinking about psychology, rather than a specific theory. Evolutionary psychology is an approach that can be applied to any topic in psychology and, in principle, other social sciences. It assumes that many human psychological adaptations are designed to solve problems of survival and reproduction in the EEA. Behavioral genetics is the study of genetic and environmental influences on behaviors. By using research methodologies from twin and adoption studies through to DNA analysis, behavioral geneticists are able to estimate the relative contributions of genetic and environmental factors to a given trait or behavior. The focus of the research is to understand why people differ and to discover the underlying causal processes. As research has shown that most behaviors have heritable components, it is now considered rare that a psychological trait is purely environmental. Highly heritable traits may nonetheless be environmentally malleable. The explosion of advances in genetic technology has increased interest in this field as it has the potential to solve many issues in public policy and the understanding of health and well-being. Psychophysiology is the branch of psychology that is concerned with the physiological bases of psychological processes. While psychophysiology was a general broad field of research in the 1960s and 1970s, it has now become quite specialized and has branched into subspecialties. In the general field, a central interest is with the relationship between physiological activity and psychological processes in humans and animals. This is often done through attempts to record electrical activity from the brain and nervous system in an effort to understand the function of the various brain systems and their possible relationship to specific psychological processes. Cognitive neuroscience is an interdisciplinary area of study that has emerged from psychophysiology. Cognitive neuroscientists are generally interested in understanding the biological substrates that underlie human cognition. It is a largely experimental field that has developed many new technologies for studying the brain. Primarily, the aim of cognitive neuroscience is to provide a framework for understanding the relationship between cognitive and mental processes and the underlying physiological mechanisms that support behavior.

3.1. Evolutionary Psychology

Evolutionary psychology is an approach to studying psychology. It applies principles of evolution, which are defined as changes in gene frequency in a population from one generation to the next, to the structure and function of the human nervous system. The approach explores and tries to explain psychological traits such as memory, perception, or language as evolved adaptations. The basic idea of evolutionary psychology is that the nervous system is a computer that processes information. It is a physical system that has input (senses), processes (thinking), and output (behaviors). The theoretical foundation has a substantial and rich empirical basis that is often provocative. Theories about normal and abnormal human motivation, emotion, and behavior are often controversial because they concern various topics that people hold dear. For example, mating behavior, racial differences, and the nature of intelligence. Another reason for it being deemed controversial is its use for generating and testing new hypotheses. These hypotheses are direct products of asking evolutionary questions about the design of the human mind. Testing them often requires venturing into new territory where the complex interplay between genes and environment and between proximate and ultimate causation can be sorted out. Evolutionary psychology is still a young theory but has prompted a large amount of research and has the potential to unify the social and natural sciences.

3.2. Behavioral Genetics

Behavioral genetics researchers study genetic influence on behavior. In some ways, behavioral genetics is the classic example of the biopsychological approach. Early work on behavioral genetics focused on areas in which biological and environmental factors could be clearly separated (e.g., studying the effects of genetics on behavior in mice, whose environment can be easily controlled) in an attempt to find ways to separate the effects of biology and environment on human behavior. This led to the realization that in some situations it is inappropriate to talk about biological and environmental factors as though they are separable entities that sum linearly to produce behavior. This perspective has opened up a significant amount of research into areas more traditionally associated with social or developmental psychology (e.g., how genetic and environmental factors interact to produce personality) from a biopsychological viewpoint. Broadly, studies in behavioral genetics can be divided into two types. The first involves finding humans who have a genetic difference that is believed to affect behavior and studying the effects of that difference in an attempt to make a genotype-phenotype connection. These studies are often difficult to conduct and require researchers to be creative in finding ways to test their hypotheses (e.g., a study involving the genetic disorder PKU found that the behavioral effects of the disease could be alleviated by a special diet. A similar study would have been impossible to conduct on healthy individuals with the same genotype). The second type of study involves estimating the heritability of a behavior. This is usually done by studying the behavior of twins or adopted children to see how the behavior differs in groups that differ genetically. Estimating heritability is typically a prelude to trying to find specific genes that are involved in a behavior.

3.3. Psychophysiology

Psychophysiology is the concern for the physiological substrates of psychological processes. It has a long history and predates rigorous scientific inquiry in the area of psychology. Indeed, its early history was as a form of vitalism where the body's 'vital forces' were thought to be in a state of balance. Any movement from this state was seen as disease and mental illness was explained in this way. Such notions still exist in the Chinese medicine constructs of acupuncture and the Indian tradition of Ayurvedic medicine. Modern psychophysiology seeks to ground psychological theory in bodily function. This can be at the systemic level examining broad effects such as increased heart rate or at the molecular level such as studying the effects of a neurotransmitter on behavior. Psychophysiology makes use of the experimental method in establishing causality and operates on the assumption that all psychological states will have a corresponding neural event. This is in contrast to the dualist view which assumes that some psychological phenomena will not be based in the physical. Although the resilience of dualism in psychiatry has meant that the vast majority of research in psychopathology has a psychophysiological flavor as findings can be fitted to either physical or psychological causes. The mainstay of psychophysiology has been the use of measures and tools from the rest of biological science. For example, the use of the ECG, measuring skin conductance, and the use of brain imaging techniques. With the availability of methods in molecular biology, psychophysiology increasingly makes use of understanding behavior through gross measures to looking at specific neurotransmitter receptor activity and the release of neuropeptides. This is perhaps the closest that any field in psychology has come to realizing the biopsychosocial model.

3.4. Cognitive Neuroscience

Cognitive neuroscience seeks to understand psychological phenomena interactively in terms of neural underpinnings and the associated mental activity. Since its origin from cognitive psychology and neuroscience, this field has elevated our understanding of cognitive and emotional processes, and contributed to the diagnosis and treatment of brain disorders. Each psychology commentator discusses research chapter 2 a little too well by subtly theorizing, suggesting theoretical bias prevails, and what higher order cognitive and emotional process is not subtly racial. This tradition is maintained by MacPhail's targeting of Piagetian theory and the discipline's adherence to positivism, which necessitates a postulation of cause and effect relationships between neurological events and behavioral and cognitive change. Cognitive neuroscience has been and continues to be important for biological psychology and some would suggest is the future focus of the discipline.

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Biological Beginnings:

Each human cell has a nucleus which contains chromosomes made up of deoxynucleic acid, or DNA. DNA contains the genetic information, or genes, that are used to make a human being. All typical cells in a human body have 46 chromosomes arranged in 23 pairs, with the exception of the egg and sperm. During cell reproduction, or mitosis, the cell’s nucleus duplicates itself and the cell divides and two new cells are formed. Meiosis is a different type of cell division in which eggs and sperm, or gametes, are formed. During meiosis, a cell duplicates its chromosomes, but then divides twice, resulting in a cell with 23 unpaired chromosomes. During fertilization, an egg and sperm combine to form a single cell, zygote, with information from both the mother and the father.

The combination of the unpaired chromosomes leads to variability in the population because no two people are exactly alike, even in the case of identical twins. A person’s genetic make-up is called their genotype; this is the basis for who you are on a cellular level. A person’s phenotype is what a person’s observable characteristics are. Each genotype can lead to a variety of phenotypes. There are dominant and recessive genes contained in the genetic material that we acquire. For example, brown eyes are dominant over blue eyes, so if the genetic code is available for both, brown eyes will prevail.

Abnormalities can also be linked to the chromosomes and genes that are inherited from your parents. Some examples are down syndrome, cystic fibrosis, and spina bifida. This is caused when either chromosomes or genes are missing, mutated or damaged.

Genetically, I received my height and brown eyes from my mother, and my brown hair from both my parents. As far as I know, I don’t have any abnormalities linked to my chromosomes or genes that were passed down during my conception.

Prenatal/Post-partum:

The prenatal stage starts at conception, lasts approximately 266 days, and consists of three different periods: germinal, embryonic and fetal. This is an amazingly complex time that allows a single cell composed of information from both the mother and the father to create a new human being.

The first period of the prenatal stage occurs in the first two weeks after conception and is called the germinal period. During this time the zygote (or fertilized egg) begins its cell divisions, through mitosis, from a single cell to a blastocyst, which will eventually develop into the embryo and placenta. The germinal period ends when the blastocyst implants into the uterine wall.

The second period of prenatal development that occurs in weeks two through eight after conception is called the embryonic period. During this time, the blastocyst from the first stage develops into the embryo. Within the embryo, there are three layers of cells that form: the endoderm, which will develop into the digestive and respiratory systems, the ectoderm which will become the nervous system, sensory receptors and skin parts, and the mesoderm which will become the circulatory system, bones, muscles excretory system and reproductive system. Organs begin to form in this stage also. During this stage, the embryo development is very susceptible to outside influences from the mother such as alcohol consumption and cigarette usage.

The fetal period is the final period of the prenatal stage which lasts from two months post conception until birth. It is the longest period of the prenatal stage. During this period, continued growth and development occur. At approximately 26 weeks post conception, the fetus would be considered viable, or able to survive outside the mother’s womb. If birth would occur at 26 weeks, the baby would most likely need help breathing at this point because the lungs are not fully mature, but all organ systems are developed and can function outside of mom.

The brain development during the prenatal period is also very complex, and if you think about it, an amazing thing. When a baby is born, they have 100 billion neurons that handle processing information. There are four phases of brain development during the prenatal period: formation of the neural tube, neurogenesis, neural migration and neural connectivity.

During the prenatal period, a wide variety of tests can be performed to monitor the development of the fetus. The extent to which testing is used, depends on the doctors’ recommendations as well as the mothers age, health and potential genetic risk factors. One common test utilized is the ultrasound. This is a non-invasive test that is used to monitor the growth of the fetus, look at structural development and determine the sex of the baby. Other tests that are available, but are more invasive and riskier for both the fetus and the mother, include Chorionic villus sampling, amniocentesis, fetal MRI, maternal blood screening.

The mother’s womb is designed to protect the fetus during development. However, if a mother doesn’t take care of herself, it can have a negative impact on the developing fetus. A woman should also avoid x-rays and certain environmental pollutants during the pregnancy. A woman should avoid alcohol, nicotine, caffeine, drugs and teratogens. They should also have good nutrition during the pregnancy as the fetus relies solely on the mother for its nutrients during development. Along with good nutrition, extra vitamins are also recommended during the pregnancy period, the main one recommended in folic acid. Emotional health is also very important. Higher degrees of anxiety and stress can also be harmful to the fetus and have long term effects on the child.

The birth of a child marks the transition from the prenatal to post-partum stage, which last from approximately 6 weeks, or until a mother’s body is back to her pre-pregnancy state. During this time a woman may be sleep deprived due to the demands of the baby and trying to take care of any other family members. There are also hormonal changes that woman experiences as well as the uterus returning to its normal size. Emotional adjustments are also occurring during this stage. It is common for most women to experience the post-partum blues, in which they feel depressed. These feelings can come and go, and usually disappear within a couple of weeks. If major depression occurs beyond this time, this is referred to as postpartum depression and it is important for a woman to get treatment to protect herself and her baby.

My prenatal development and delivery were fairly uneventful for my mother. The only complication that was had during her pregnancy was low iron levels which would cause her to pass out. Once she started on iron pills, this problem was eliminated. During her pregnancy, since it was in the early 1970’s, it wasn’t common for any testing or ultrasounds to occur unless there were major complications. As my mom said, you get pregnant and have a baby. After I was born, my mom said that she had no complications from post-partum depression or baby blues.

Infancy is the period of time between birth and two years of age. During this time, extraordinary growth and development occur following a cephalocaudal pattern (or top down) and a proximodistal pattern (center of body to extremities). A baby can see before it speaks, move its arms before its fingers, etc. An infant’s height increased by approximately 40 percent by the age of 1. By the age of 2, a child is nearly one-fifth of the its weight and half its height as they will be as an adult. Infants require a great deal of sleep, with the average in this period of 12.8 hours a day. The sleep an infant gets can have an impact of their cognitive functions later in life, such as improved executive function (good sleep) or language delays (poor sleep).

Proper nutrition during this period is also imperative for infant development. Breast feeding an infant exclusively during the first six months of life provides many benefits to both the infant and the mother including appropriate weight gain for the infant and a reduction in ovarian cancer for the mother. However, both breast feeding and bottle feeding are appropriate options for the baby. As the infant gets older, appropriate amounts of fruits and vegetables are important for development as well as limiting junk food.

Motor skills development is thought to follow the dynamic systems theory in this the infant assembles skills based on perceptions and actions. For example, if an infant wants a toy, he needs to learn how to reach for that toy to grasp it. An infant is born with reflexes, which are required for them to adapt to their environment before they learn anything, such as the rooting reflex and sucking reflex for eating. Some of these reflexes are specific to this age, some are permanent throughout their life, such as blinking of the eyes. Gross motor skills are the next major skill that an infant develops. These involve the large muscle groups and are skills such as holding their head up, sitting, standing, and pulling themselves up on furniture. The first year of life, the motor skills help the infant provide themselves independence, while the second year is key to honing in the skills they have learned. Fine motors skills develop secondary to gross motor skills. These include activities such as grasping a spoon and picking up food off of their high-chair tray.

Infant senses are not developed during the prenatal period. Visual acuity in the infant that is comparable to an adult, occurs by about 6 months of age. A fetus can hear in the womb, but is unable to distinguish loudness and pitch which is developed during infancy. Other senses are present, such as taste and smell, but preferences are developed throughout infancy.

Jean Piaget’s theory on cognitive development is one that is widely used. This theory stresses that children develop their own information about their surroundings, instead of information just being given to them. The first stage of Piaget’s theory is the sensorimotor stage. This stage involves infants using their senses to coordinate with their motor skills they are developing. There is some research that has been completed that states that Piaget’s theories may need to be modified. For example, Elizabeth Spelke endorses a core knowledge approach, in which she believes that infants are born with some innate knowledge system in order for them to navigate the world in which they are born into.

Language development begins during this stage also and all infants follow a similar pattern. The first sounds from birth is babbling, crying and cooing which are all forms of language. First words are usually spoken by about 13 months with children usually speaking two word sentences by about two years. Language skills can be influenced both by biological and environmental considerations in the infant.

An infant displays emotion very early in life. The first six months of their life you can see surprise, joy, anger, sadness, and fear. Later in infancy, you will also see jealousy, empathy, embarrassment, pride, shame and guilt. The later developed emotions are emotions that require thought, which is why they don’t develop until after the age of 1. Crying can indicate three different emotions in an infant – basic cry – typically related to hunger, anger cry and pain cry. A baby’s smile can also mean different things – such as a reflexive smile or a social smile. Fear is an emotion that is seen early in a baby’s life. One that is often talked about is “stranger danger” or separation protest.

There are three classifications of temperaments of a child that were proposed by Chess and Thomas. These include an easy child, difficult child, and slow-to-warm up child. These temperaments can be influenced by biology, gender, culture and parenting styles. The remaining personality traits that are developed in the period include trust, developing sense of self and independence. Erik Erickson first stage of development occurs within the first year of life with his trust vs mistrust theory. The concept of trust vs mistrust is seen throughout the development of a person and is not limited to this age group. The second year of life Erickson’s theory of autonomy vs shame and doubt. As an infant develops his skills, they need to be able to do this independently or feelings of shame and doubt develop. The development of autonomy during infancy and the toddler years can lead to greater autonomy during the adolescent years.

Social interactions occur with infants as early as 2 months of age, when they learn to recognize facial expressions of their caregivers. They show interest in other infants as early as 6 month of age, but this interest increases greatly as they reach their 2nd birthday. Locomotion plays a big part in this interaction allowing the child to independently explore their surrounding and others that may be around them. Attachment theories are widely available. Freud believes attachment is based on oral fulfillment, or typically the mother who feeds them. Harlow said that attachment is based on the comfort provided based on his experiment with wire monkeys. Erikson’s theory goes back to the trust vs mistrust theory which was talked about earlier.

As a new baby is brought into a family, the dynamic of the household changes. There is a rebalancing of social, parental and career responsibilities. The freedom that was once had prior to the baby is no longer there. Parents need to decide if a parent stays home to take care of the child or if the child is placed into a daycare setting. Parental leave allows a parent to stay home with their child for a period of time after their birth, but then requires them to be place in some type of child care setting. Unfortunately, the quality of child care varies greatly. Typically, the higher the quality, also the higher the price tag. A parent needs to be an advocate for their child and monitor the quality of care they are receiving, no matter the location they are at. There has been shown to be little effect on the children who are placed in child care instead of being cared for by a full-time parent.

As an infant, I was a bottle-fed baby. My mother was able to be home with me full time, so I was not exposed to outside childcare settings. Unfortunately for my parents, I was very colicky until I was about 6 weeks old. This was very stressful for my parents as they were adjusting to life as a family with a new baby. After the colic ended, I was a very happy, easy baby when I wasn’t sick. I developed febrile seizures about the age of 7 months and they lasted until about 2 years when I was put on phenobarbital to control them. I talked and walked at a very young age (~9 months). I was very trusting of everyone and had no attachment issues. I was happy to play by myself if no one was around, but if company was over, my parents said I always wanted to be in the middle of the action, I was especially fond of adult interactions.

Early childhood:

The next developmental stage is early childhood which lasts from around the ages of 3-5. During this stage, height and weight are slowed from the infancy stage, but a child still grows about 2 ½ inches a year and gains 5-7 pounds per year during this stage. The brain continues to develop by combining the maturation of the brain with the external experiences. During this stage, increased. The size of the brain doesn’t increase dramatically during this or subsequent periods, but the local patterns within the brain do. The most rapid growth occurs in the prefrontal cortex which is key in planning and organization as well as paying attention to new tasks. The growth during this phase is caused by the increase in the number and size of the dendrites as well as the myelination continuing during this stage.

Gross motor skills continue to increase with children being able to walk easily as well as beginning movements of hopping, skipping, climbing, etc. Fine motor skills continue to improve as well with children being able to build towers of blocks, do puzzles, or writing their name.

Nutrition is an important aspect in early childhood. Obesity is a growing health problem in early childhood. Children are being fed diets that are high in fats and lower in nutritional value. They are eating out more that they have historically. Parents need to focus on better nutrition and more exercise for their children. Childhood obesity has a strong correlation to obesity later in life.

Piaget’s preoperational stage lasts from age 2 to 7, is the second stage in his theory of development. During this stage, children begin to represent things with words, images and drawings. They are egocentric and hold magical beliefs. This stage is divided into the symbolic function substage (age 2-4) and the intuitive thought substage (age 4-7). In the symbolic substage, the child is able to scribble designs the represent objects, and can engage in pretend play. They are limited in this stage by egocentrism and animism. The intuitive thought substage, the child begins to use primitive reasoning and is curious. During this time, memory increases as well as their ability to improve their attention span.

Language development during this phase is great. A child goes from saying two word utterances, to multiple word combinations, to complex sentences. They begin to understand the phonology of language as well as the morphology. They start to also apply the rules of syntax and semantics. The foundation for literacy also begins during this stage, using books with preschoolers provides a solid foundation for which the rest of their life successes can be based.

There are many early childhood education options available to parents. One option is the child centered kindergarten which focuses on the whole child. The Montessori approach allows the children more freedom to explore and the teacher is a facilitator rather than an instructor. There are also government funded programs such as Project Head Start available for low-income families to give their children the experience they need before starting elementary school.

Erickson’s stage of development for early childhood is initiative vs guilt. This stage the child has begun to develop an understanding of who they are, but also begin to discover who they will become. Usually children of this age describe themselves in concrete terms, but some also begin to use logic and emotional descriptors. Children also begin to perceive others in terms of psychological traits. Children begin to be more aware of their own emotions, understand others emotions and how they relate to them and are also able to begin regulating emotions during this stage.

Moral development also begins during this stage. Freud talks about the child developing the superego, the moral element of personality, during this stage. Piaget said children go through two distinct stages of moral reasoning: 1) heteronomous morality and 2) autonomous morality. During the first, the child thinks that the rules are unchangeable and judge an action by the consequence, not the intention. The autonomous thinker thinks about the intention as well as the consequence.

Gender identity and roles begin to play a factor during this stage. Social influences on gender roles provide a basis for how children think. This can be through imitation of what they see their parent doing or can be through observation of what they see around them. Parental and peer influences on modeling behavior is apparent. Group size, age, interaction in same-sex groups and gender composition all are important aspect of peer relations and influences.

Parenting style vary differently. Diana Baumrind describes four parenting styles in our book: authoritarian, authoritative, neglectful and indulgent. She shows a correlation between the different parenting styles and the behaviors in children.

Play is important in the child’s cognitive and socioemotional development. Play has been considered the child’s work by Piaget and Vygotsky. This allows a child to learn new skills in a relaxed way. Make-believe play is an excellent way for children to increase their cognitive ability, including creative thought. There are many way a child can play – including sensorimotor and practice play, pretense/symbolic play, constructive play, and games. Screen time is becoming more of a concern in today’s world. They are good for teaching, but can also be distracting/disruptive if screen time is not limited.

As a young child, I was very curious about thing and loved to play pretend. I attended preschool for two years, which aided in my cognitive development. My parents said I was able to read and do age advanced puzzles by the time I was 3. I was able to regulate my emotions and understand the emotions of others. My parents utilized an authoritarian style of discipline when I was younger, being the first child, they wanted their kids to be perfect. This relaxed as my siblings came along and as we got older.

Middle/late childhood:

During this period, children maintain slow, consistent physical growth. They grow 2-3 inches per year until the age of about 11, and gain about 5-7 pounds per year. The size of the skeletal and muscular systems is the main contributor to the weight gain.

The brain volume stabilizes by the end of this stage, but changes in the structures continue to occur. During this stage, there is synaptic pruning, in which some areas of the brain which are not used as frequently lose connection, while other areas increase the amount of connections. This increase is seen in the prefrontal cortex which orchestrates the function of many other brain regions.

Development of both gross and fine motor skills continue to be refined. Children are able to ride a bike, swimming, skipping rope; they can tie their shoes, hammer a nail, use a pencil and reverse numbers less often. Boys usually outperform girls in their gross motor skills, while girls outperform boys in the fine motor skills. Exercise continues to be area of concern at this age. Children are not getting the exercise they need. Studies have shown that aerobic exercise, not only helped with weight, but also with attention, memory, thinking/behavior and creativity.

Obesity is a continued health concern for this age group which leads to medical concerns such as hypertension, diabetes, and elevated cholesterol levels. Cancer is the second leading cause of death of children in this age group. The most common childhood cancer is leukemia.

Disabilities are often discovered during this time as many don’t show up until a child is in a school setting. There are learning disabilities, such as dyslexia, dysgraphia and dyscalculia; attention deficit hyperactivity disorder (ADHD), and autism spectrum disorders, such as autistic disorder and Asperger syndrome. Schools today are better equipped to handle children with these disabilities to help them receive the education they need.

This stage of development as described by Piaget Cognitive Development Theory is the concrete operational stage. The child in this stage can reason logically, as long reasoning can be applied to concrete examples. In addition, they can utilize conservation, classification, seriation and transitivity.

Long term memory increases during this stage, in part in relation to the knowledge a child has with a particular subject. Children are able to think more critically and creatively during this period as well as increases in their metacognition. Along with the topics already mentioned: self-control, working memory and flexibility are all indicators of school readiness/success.

Changes occur during this stage on how a child’s mental vocabulary is organized. They begin to improve their logical reasoning and analytical abilities. They also become have more of metalinguistic awareness, or knowledge about language. Reading foundations are important during this stage. Two approaches currently being explored are the whole-language approach and the phonics approach. The whole-language approach teaches children to recognized words or whole sentences. The phonic approach teaches children to translate written symbols into sounds.

The child during this stage, begins to better understand themselves and are able to describe themselves utilizing psychological characteristics and can describe themselves in reference to social groups. High self-esteem and self-concept are important for this age group. Low self-esteem has been correlated to instances of obesity, depression, anxiety, etc.

Erickson’s fourth stage of development, industry vs inferiority appears in this stage. Industry refers to work and children wanting to know how things are made and how they work. Parents who dismiss this interest can create a sense of inferiority in their children.

Emotional development during this stage involves the child becoming more self-regulated in their reactions. They understand what lead up to an emotional reaction, can hide negative reactions and can demonstrate genuine empathy. They are also learning coping strategies to learn to deal with stress. Moral development continues also during this stage as proposed by Kohlberg’s 6 stages of development.

Gender stereotypes are prevalent in this development phase. They revolve around physical development, cognitive development and socioemotional development of a child.

This stage of life, parents are usually less involved with their children although they continue to remain an important part of their development. They become more of the manager, helping the child learn the rights/wrongs of their behaviors. If there is a secure attachment between the parent and the child, the stress and anxiety that is involved in this phase is lessened.

Friendships are important during this stage of a child’s life. Friends are typically similar to the child in terms of age, sex, and attitudes towards school. School is a sign of new obligations to children. As with the younger age group, there are different approaches to school at this stage. A constructivist approach focuses on the learner and having the individuals constructing their knowledge. A direct instruction approach is more structured and teacher centered. Accountability in the schools is enforced through the application of standardized testing. Poverty plays a role in the learning ability of children, oftentimes creating a barrier to learning for the student, including parents with low expectations, not able to help with the homework of inability to pay for educational materials.

My parents said that by this age I was able to reason logically with them, and in my day to day life. I remained curious about what things were and how they worked. My mom told me about a test I took for an accelerated learning program (ULE) in my elementary school. I missed one question, I couldn’t answer what a wheelbarrow was. After that, my mom said I was interested in learning what they were and what they were used for. The ULE program helped me satisfy my curiosity above and beyond what was taught in school by providing additional learning opportunities.

Adolescence:

Adolescence lasts from about 12 – 18 years of age. The primary physical change during adolescence is the start of puberty. This is a brain-neuroendocrine process that provides stimulation for rapid physical changes that take place. This is when a child takes on adult physical characteristic, such as voice changes, height/weight growth for males and breast development and menstruation begins for females. Females typically enter puberty two years prior to males. The process is hormonal driven and include actions from the hypothalamus and pituitary gland. During this time, adolescents are preoccupied with their body image, as their bodies are rapidly changing. Females are typically more dissatisfied with their bodies than males, however, body image perception becomes more positive for both genders as they end the adolescent period.

Brain development during this time includes significant structural changes. The corpus callosum thickens, improving their ability to process information. The prefrontal lobes continue to develop, increasing reasoning, decision making and self-control. The limbic system, specifically the amygdala is completely developed by this stage.

This stage also marks a time of sexual exploration, forming a sense of sexual identity, managing sexual feelings, and developing intimate relationships. Most adolescents are not emotionally prepared for sexual experiences and can lead to high risk sexual factors. Contraceptive use is not prevalent in this age group, even though it can lessen or eliminate the risk of sexually transmitted diseases and unwanted pregnancy. Teen pregnancy, while reduced from years past, is still too high. Sex education continues to be a topic of discussion as to what is most appropriate for the schools – abstinence only or education that emphasizes contraceptive knowledge.

Health during this stage of development is of concern as bad health habits learned here, can lead to death in early adult life. Obesity due to poor nutrition and lack of exercise remains a consistent theme. Sleep is also important for this age group as most reported getting less than 8 hours of sleep per night. Substance use is also seen in this age group. Another health concern is eating disorders including both anorexia and bulimia, these disorders can take over a person’s life due to distorted body images.

Piaget’s final stage of cognitive development occurs during this stage – the formal operational stage. Adolescents are not bound by concrete thoughts or experiences during this stage. They can think abstractly, idealistically, and logically.

Executive function is one of the most important cognitive changes that occurs in this stage. This involves an adolescent ability to have goal directed behavior and the ability to exercise self-control.

The transition between elementary school to junior high school during this stage can be very stressful for adolescents. It occurs during a period of time when many other physical changes (puberty) are occurring at the same time. This can create stress and worrying for the child.

Erickson’s fifth developmental stage that corresponds to this period in life is Identity vs identity confusion. This stage is aided by a psychosocial moratorium, which is the gap between adolescence and adulthood. This period a person is relatively free of responsibility to determine what their true identity is. This is the path that one takes toward adult maturity. Crisis during this stage is a period in which a person is identifying alternatives. Commitment is a personal investment in an identity. It is believed that while identity is explored during this stage, finalization does not occur until early adulthood, with life review.

Parents take on a managerial role during this stage; monitoring the choices that are made regarding friends, activities, and their academic efforts. Higher rates of parental monitoring leads to lower rates of alcohol and drug use. The adolescents need for autonomy can be hard for a parent to accept. The parents feel like the child is “slipping away” from them. There is also gender differences as far as it relates to how much autonomy is granted, with males receiving more autonomy than females. Conflict during this escalates during the early adolescent stage, but then lessens towards the end of the stage.

Friendships during this stage are often fewer, but more intimate than in younger years and take on an important role of meeting social needs. Positive friendships are associated with positive outcomes, including lower rates of substance abuse, risky sexual behavior, bullying and victimization. Peer pressure at this stage in life is high, with more conformance to peer pressure if they are uncertain about their social identity. Cliques and crowds emerge and provide a more important role during this stage of development. Dating and romantic relationships begin to evolve. Juvenile delinquency is a problem that emerges, with illegal behaviors being noted. This can be due to several factors including lower socioeconomic status, sibling relationships, peer relationships, and parental monitoring. Depression and suicide also increase during this stage of life.

During this stage of my life, I was very goal oriented, more so academically than socially. I chose to take higher level classes that weren’t required and continued to work with a program that allowed me to do projects outside of school. During this time, I began to think about what direction my life would take. I decided that I would attend college to major in pharmacy, a decision that would later be reviewed and changed.

Early adulthood:

Becoming an adult involves a lengthy transition. Early adulthood occurs from 18 to 25 years of age. During this time, an individual is still trying to figure out “who” they are, exploring career paths, determining their identity and understanding what kind of lifestyle they want to live. Early adulthood is characterized by 5 key features as explained by Jeffrey Arnett. These include: identity exploration, instability, self-focused, feeling in-between and the age of possibilities – basically they can transform their lives. In the US, entry into adulthood is primarily characterized by holding a permanent, full-time job. Other countries consider marriage the marker for adulthood. Just as going from elementary school to middle school causes stress in adolescents, the transition from high school to college can evoke the same emotions.

Peak physical performance is often reached between the ages of 19 and 26. Along with physical performance decline, body fatty tissue increases and hearing begins to decline in the last part of early adulthood. Health during early adulthood is subpar. Although most know what is required to be healthy, many fail to apply this information to themselves. The bad habits started during adolescence are increased in early adulthood, including inactivity, diet, obesity, sleep deprivation and substance abuse. These lifestyles, along with poor health, also have an impact on life satisfaction. Obesity continues to be a problem in this developmental stage. Losing weight is best achieved with a diet and exercise program rather than relying on diet alone. Exercise can help prevent diseases such as heart disease and diabetes. Exercise can also improve mental health as well and has been effective in reducing depression. Alcohol use appears to decline by the time an individual reaches their mid-twenties, but peaks around 21-22 years of age. Binge drinking and extreme binge drinking are a concern on college campuses. This can lead to missing classes, physical injuries, police interactions and unprotected sex.

Sexual activity increases in emerging adulthood, with most people having experienced sexual intercourse by the time they are 25. Casual sex is common during this development stage involving “hook-ups” or “friends with benefits”.

Piaget’s stages of development ended with formal operational thought that was discussed in the adolescent stage. However, he believes that this stage covers adults as well. Some theorists believe that it is not until adulthood that formal thoughts are achieved. An additional stage has been proposed for young adults – post-formal thought. This is reflective, relativistic, contextual, provisional, realistic and influenced by emotion.

Careers and work are an important theme in early adulthood. During this time an individual works to determine what career they want to pursue in college by choosing a major. By the end of this developmental stage, most people have completed their training and are entering the work force to begin their career. Determining one’s purpose, can help ensure that the correct field of study/career choice is made. Work defines a person by their financial standing, housing, how they spend their time, friendships and their health. Early jobs can sometimes be considered “survival jobs” that are in place just until the “career job” is obtained.

Erickson’s sixth stage of development that occurs during early adulthood is intimacy vs isolation. Intimacy, as described by Erickson, is finding oneself while losing oneself in another person, and it requires a commitment to another person. Balancing intimacy and independence is challenging. Love can take on multiple forms in adulthood. Romantic love, or passionate love, is the type of love that is seen early in a relationship, sexual desire is the most important ingredient in romantic love. Affectionate love, or compassionate love, is when someone desires to have the other person near and has a deep, caring affection for the person, typically a more mature love relationship. Consummate love involves passion, intimacy and commitment, and is the strongest of all types of love.

Adults lifestyles today are anything but conventional. Many adults choose to live alone, cohabitate, or live with a partner of the same sex in addition to the conventional married lifestyle. Divorce rates continue to remain high in the US, with most marriages ending early in the course of their marriage. Divorced adults have higher rates of depression, anxiety, suicide, alcoholism and mortality. Adults who remarry usually do so within three years of their divorce, with men remarrying sooner than women. Making a marriage work takes a great deal of commitment from both parties. John Gottman determined some principals that will help make a marriage successful. These include: establishing love maps, nurturing fondness and admiration, turning towards each other instead of away, letting the partner influence you and creating shared meaning. In addition, to these a deep friendship, respect for each other and embracing the commitment that has been made are things that will help to make a marriage last.

During early adulthood, many become parents for the first time. Sometimes this is well planned out, other times it is a complete surprise. Parenting is often a hybrid of utilizing techniques that their parents used on them and their own interpretation of what is useful. The average age an individual has their first child is increasing and the number of children they choose to have is declining. This is due to women wanting to establish their careers prior to becoming a mom. The results of this is that parents are often more mature and able to handle situations more appropriately, may have more income, fathers are more involved in the child rearing but also children spend more time in supplemental care than when mothers stayed home to provide the child care.

During early adulthood, I went to college, decided that a pharmacy major wasn’t for me and ended up obtaining a degree in microbiology and a minor in chemistry. I met my first husband during college and we ended up marrying a couple months before I graduated. After graduation, we had a child and eventually ended up getting a divorce. I think the stress of going right from college to marriage to having a family took a toll on us. We were able to maintain civility to co-parent our son even though we were not able to make our marriage work. The first few years after our divorce were very hard, being a single-mom, trying to get a career established and make sure I was providing for our child. Thankfully I had a huge support system with my parents and siblings that were able to get us through the tough times. About 10 years later, I met my now husband and was able to find the intimacy again that was needed in my life. We both have brought children from previous relationships into our marriage and have also had two children together. This has created some conflict of its own, but we work through it all together. I feel that we are much more equipped and mature to be parents of our younger children than we were when our older ones were little.

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National Research Council (US) and Institute of Medicine (US) Forum on Adolescence; Kipke MD, editor. Adolescent Development and the Biology of Puberty: Summary of a Workshop on New Research. Washington (DC): National Academies Press (US); 1999.

Adolescent Development and the Biology of Puberty: Summary of a Workshop on New Research.

• Hardcopy Version at National Academies Press

Adolescent Development and the Biology of Puberty

Adolescence is one of the most fascinating and complex transitions in the life span. Its breathtaking pace of growth and change is second only to that of infancy. Biological processes drive many aspects of this growth and development, with the onset of puberty marking the passage from childhood to adolescence. Puberty is a transitional period between childhood and adulthood, during which a growth spurt occurs, secondary sexual characteristics appear, fertility is achieved, and profound psychological changes take place.

Although the sequence of pubertal changes is relatively predictable, their timing is extremely variable. The normal range of onset is ages 8 to 14 in females and ages 9 to 15 in males, with girls generally experiencing physiological growth characteristic of the onset of puberty two years before boys. Pubertal maturation is controlled largely by complex interactions among the brain, the pituitary gland, and the gonads, which in turn interact with environment (i.e., the social, cultural, and ambient environment). A relatively new area of research related to puberty is that of brain development. Evidence now suggests that brain growth continues into adolescence, including the proliferation of the support cells, which nourish the neurons, and myelination, which permits faster neural processing. These changes in the brain are likely to stimulate cognitive growth and development, including the capacity for abstract reasoning.

Although the biology of physical growth and maturation during puberty is generally understood, available data on the biochemical and physiological mediators of human behavior are extremely primitive, and their clinical applicability remains obscure. Despite the limitations of available data, a substantial body of evidence suggests that variations in the age of onset of puberty may have developmental and behavioral consequences during adolescence. Mounting evidence also suggests that gonadal hormones, gonadotropins, and adrenal hormones influence and are affected by social interactions among groups of experimental animals, and they may also play an important role in regulating human social behavior. Interesting and potentially informative parallels exist between the maturational process in human beings and in other animals, especially those having well-documented social structures.

Research conducted with both humans and nonhuman primates suggests that adolescence is a time for carrying out crucial developmental tasks: becoming physically and sexually mature; acquiring skills needed to carry out adult roles; gaining increased autonomy from parents; and realigning social ties with members of both the same and the opposite gender. Studies of such commonalities underscore the critical importance of this part of the life course in establishing social skills. For many social species, such skills are further developed through peer-oriented interactions that are distinct from both earlier child-adult patterns and later adult pairings.

Adolescence is a time of tremendous growth and potential, but it is also a time of considerable risk. Most people would argue that being an adolescent today is a different experience from what it was even a few decades ago. Both the perceptions of this change and the change itself attest to the powerful influence of social contexts on adolescent development. Many of the 34 million adolescents in the United States are confronting pressures to use alcohol, cigarettes, or other drugs and to initiate sexual relationships at earlier ages, putting themselves at high risk for intentional and unintentional injuries, unintended pregnancies, and infection from sexually transmitted diseases (STDs), including the human immunodeficiency virus (HIV). Many experience a wide range of painful and debilitating mental health problems.

One of the important insights to emerge from scientific inquiry into adolescence in the past decade is the profound influence of settings on adolescents' behavior and development. Until recently, research conducted to understand adolescent behavior, particularly risk-related behaviors, focused on the individual characteristics of teenagers and their families. In 1993, the National Research Council conducted a study that took a critical look at how families, communities, and other institutions are serving the needs of youth in the United States. This study concluded that adolescents depend not only on their families, but also on the neighborhoods in which they live, the schools that they attend, the health care system, and the workplace from which they learn a wide range of important skills. If sufficiently enriched, all of these settings and social institutions in concert can help teenagers successfully make the transition from childhood to adulthood.

Family income is perhaps the single most important factor in determining the settings in which adolescents spend their lives. Housing, neighborhoods, schools, and the social opportunities that are linked to them are largely controlled by income; a family's income and employment status decide its access to health care services and strongly influence the quality of those services (National Research Council, 1993). Opportunities for advanced education and training and entry into the workforce are also closely linked to family income. Moreover, income is a powerful influence in shaping what is arguably the most important setting, the family. At this point in time, the evidence is clear—persistent poverty exacts a significant price on adolescents' health, development, educational attainment, and socioeconomic potential, even though the causal relationships are not well understood in all cases.

Not only is current research attempting to more fully characterize the physiological mechanisms responsible for initiating and regulating neuroendocrine maturation and somatic growth, but it is also attempting to characterize these environmental and contextual factors that may interact with biological ones to enhance or impede maturation. This research is attempting to address questions that could help to inform the development of policies and the delivery of services for youth. Such questions include: What is the pubertal experience like for teenagers today, and how does it differ from that in the past, both in the United States and in other cultures? How do pubertal experiences, in some circumstances and for some subgroups, trigger maladaptive responses? What role do pubertal processes play in cognitive change? How does puberty, in conjunction with other events that occur during early adolescence, influence the emergence of developmental psychopathology?

• Cite this Page National Research Council (US) and Institute of Medicine (US) Forum on Adolescence; Kipke MD, editor. Adolescent Development and the Biology of Puberty: Summary of a Workshop on New Research. Washington (DC): National Academies Press (US); 1999. Adolescent Development and the Biology of Puberty.
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Biological Psychology: Development and Theories Essay

Introduction.

Biology and psychology help researchers, medical professionals and psychologists to understand the behavior of human beings and animals. Therefore, biological psychology is used to examine the behavior of the humans and animals in order to facilitate in the treatment of the brain. Biological psychology is also referred to as behavioral neural science, biopsychology, clinical neuropsychology, and physiological psychology. Biological psychologists use the knowledge that they acquire from psychology to help them treat mental illness cases efficiently.

They also focus on issues related to mental processes and the manner in which they are initiated in the brain (Chavez, 2009). The goal of this paper therefore is to define biological psychology, discuss the historical development of biological psychology, stipulate important theorists who are associated with biological psychology, show the relationship between biological psychology and other fields of psychology and neuroscience, and discuss the major assumptions of that are associated with bio-psychology.

Biological Psychology

Bio-psychology studies human emotional and affective processes thereby helping biological psychologists to devise ways of assisting people to cope with different mental problems. It focuses on the brain and the manner in which it influences the performance of the entire nervous system. In this case, it focuses on people’s ability to think, feel, learn, perceive and sense. Studies reveal that these characteristics are similar in humans and animals (Hubpages Inc, 2012). It also focuses on the biological processes that influence normal and abnormal behavior in humans and animals.

Historical Development of Biological Psychology

The environment influences the evolution process of human beings and animals. As the state of the environment changes, the behavior of human beings and animals also changes in order to help them adapt to their new surroundings. As a result, it is true that biology and psychology work together to help people understand the relationship between human and animal behavior. Therefore, the idea of bio-psychology was first put into practice during the Greek era. It was adopted between the 18h and 19 th centuries. In this case, Plato proposed that the brain is the vital organ that facilitates reasoning. On the other hand, Descartes stipulated that the mind and body work in a different manner. He argued that the mind is non-physical and that it influences behavior among human beings and animals (Pinel, 2009).

Researchers and theorists who supported biological psychology made it possible for people to understand mental illnesses deeply and how they influence people’s behaviors. However, when the concept of biological phycology was proposed, many people argued that it would not be possible to understand how the brain works without touching and testing it. As a result, animals were dissected and tested in order to help people understand the complexities that are found in the brain. Without testing the brain, it would not be possible for people to understand how the brain works. Today, many psychologists, researchers and doctors are working hard in order to help them treat those people who have mental problems (Lee, 2011). However, they cannot manage to treat people who have mental problems if they do not understand the functions of the brain thoroughly.

Theorists Associated With Biological Psychology

The major theorists associated with biological psychology are Rene Descartes, Thomas Willis and Luigi Galvani. Descartes believed that the flow of animal spirits influences their behavior. He also believed that human beings follow the same trend. On the other hand, Thomas Willis stipulated that the structure of the brain influences the behavior of human beings and animals. He is associated with the discovery of the white and gray matter that is present in the brain. Moreover, Luigi Galvani stipulated that the nervous tissues are powered by electricity (Lee, 2011).

Relationship Between Biopsychology and Other Fields of Psychology and Neuroscience

Biopsychology is a vital area of study because it supplies information to all fields of psychology. This is because all fields of psychology focus on the study of behaviors and the functions of the brain. Moreover, studies show that biological psychologists study cognitive neuroscience, evolutionary psychology, and neuropsychology. For example, biopsychology analyzes behavioral problems and the manner in which they influence the performance of the brain and the nervous system (Hubpages Inc, 2012). Other fields of psychology and neuroscience follow the same trend. Therefore, it is true that biopsychology is related to psychology and neuroscience.

Assumptions Associated With Biopsychology

Studies reveal that social, psychological and biological factors influence the mental and physical wellbeing of a person. As a result, there are various assumptions that support the validity of biopsychology. There are two assumptions which govern biopsychology. The first assumption stipulates that mental processes influence biological processes (Chavez, 2009). On the other hand, biological processes influence mental processes. Therefore, it is true that both mental and biological processes are related to each other.

From the analysis therefore, it is true that biopsychology is a branch of psychology. Biopsychology adopts biological concepts in order to explain animal and human behavior. However, if medical professionals, researchers and psychologists fail to develop a better understanding of the human mind, the term biopsychology will seize to exist. Therefore, it is true that human beings should learn more about the brain so that they can be able to address the complexities of the brain efficiently.

Chavez, C. H. (2009). What is biological psychology? Web.

Hubpages Inc. (2012). Biological Psychology Definition . Web.

Lee, J. (2011). Biopsychology. Web.

Pinel, J. P. (2009). Biopsychology. Boston MA: Allyn and Bacon.

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Guest Essay

The Problem With Saying ‘Sex Assigned at Birth’

By Alex Byrne and Carole K. Hooven

Mr. Byrne is a philosopher and the author of “Trouble With Gender: Sex Facts, Gender Fictions.” Ms. Hooven is an evolutionary biologist and the author of “T: The Story of Testosterone, the Hormone That Dominates and Divides Us.”

As you may have noticed, “sex” is out, and “sex assigned at birth” is in. Instead of asking for a person’s sex, some medical and camp forms these days ask for “sex assigned at birth” or “assigned sex” (often in addition to gender identity). The American Medical Association and the American Psychological Association endorse this terminology; its use has also exploded in academic articles. The Cleveland Clinic’s online glossary of diseases and conditions tells us that the “inability to achieve or maintain an erection” is a symptom of sexual dysfunction, not in “males,” but in “people assigned male at birth.”

This trend began around a decade ago, part of an increasing emphasis in society on emotional comfort and insulation from offense — what some have called “ safetyism .” “Sex” is now often seen as a biased or insensitive word because it may fail to reflect how people identify themselves. One reason for the adoption of “assigned sex,” therefore, is that it supplies respectful euphemisms, softening what to some nonbinary and transgender people, among others, can feel like a harsh biological reality. Saying that someone was “assigned female at birth” is taken to be an indirect and more polite way of communicating that the person is biologically female. The terminology can also function to signal solidarity with trans and nonbinary people, as well as convey the radical idea that our traditional understanding of sex is outdated.

The shift to “sex assigned at birth” may be well intentioned, but it is not progress. We are not against politeness or expressions of solidarity, but “sex assigned at birth” can confuse people and creates doubt about a biological fact when there shouldn’t be any. Nor is the phrase called for because our traditional understanding of sex needs correcting — it doesn’t.

This matters because sex matters. Sex is a fundamental biological feature with significant consequences for our species, so there are costs to encouraging misconceptions about it.

Sex matters for health, safety and social policy and interacts in complicated ways with culture. Women are nearly twice as likely as men to experience harmful side effects from drugs, a problem that may be ameliorated by reducing drug doses for females. Males, meanwhile, are more likely to die from Covid-19 and cancer, and commit the vast majority of homicides and sexual assaults . We aren’t suggesting that “assigned sex” will increase the death toll. However, terminology about important matters should be as clear as possible.

More generally, the interaction between sex and human culture is crucial to understanding psychological and physical differences between boys and girls, men and women. We cannot have such understanding unless we know what sex is, which means having the linguistic tools necessary to discuss it. The Associated Press cautions journalists that describing women as “female” may be objectionable because “it can be seen as emphasizing biology,” but sometimes biology is highly relevant. The heated debate about transgender women participating in female sports is an example ; whatever view one takes on the matter, biologically driven athletic differences between the sexes are real.

When influential organizations and individuals promote “sex assigned at birth,” they are encouraging a culture in which citizens can be shamed for using words like “sex,” “male” and “female” that are familiar to everyone in society, as well as necessary to discuss the implications of sex. This is not the usual kind of censoriousness, which discourages the public endorsement of certain opinions. It is more subtle, repressing the very vocabulary needed to discuss the opinions in the first place.

A proponent of the new language may object, arguing that sex is not being avoided, but merely addressed and described with greater empathy. The introduction of euphemisms to ease uncomfortable associations with old words happens all the time — for instance “plus sized” as a replacement for “overweight.” Admittedly, the effects may be short-lived , because euphemisms themselves often become offensive, and indeed “larger-bodied” is now often preferred to “plus sized.” But what’s the harm? No one gets confused, and the euphemisms allow us to express extra sensitivity. Some see “sex assigned at birth” in the same positive light: It’s a way of talking about sex that is gender-affirming and inclusive .

The problem is that “sex assigned at birth”— unlike “larger-bodied”— is very misleading. Saying that someone was “assigned female at birth” suggests that the person’s sex is at best a matter of educated guesswork. “Assigned” can connote arbitrariness — as in “assigned classroom seating” — and so “sex assigned at birth” can also suggest that there is no objective reality behind “male” and “female,” no biological categories to which the words refer.

Contrary to what we might assume, avoiding “sex” doesn’t serve the cause of inclusivity: not speaking plainly about males and females is patronizing. We sometimes sugarcoat the biological facts for children, but competent adults deserve straight talk. Nor are circumlocutions needed to secure personal protections and rights, including transgender rights. In the Supreme Court’s Bostock v. Clayton County decision in 2020, which outlawed workplace discrimination against gay and transgender people, Justice Neil Gorsuch used “sex,” not “sex assigned at birth.”

A more radical proponent of “assigned sex” will object that the very idea of sex as a biological fact is suspect. According to this view — associated with the French philosopher Michel Foucault and, more recently, the American philosopher Judith Butler — sex is somehow a cultural production, the result of labeling babies male or female. “Sex assigned at birth” should therefore be preferred over “sex,” not because it is more polite, but because it is more accurate.

This position tacitly assumes that humans are exempt from the natural order. If only! Alas, we are animals. Sexed organisms were present on Earth at least a billion years ago, and males and females would have been around even if humans had never evolved. Sex is not in any sense the result of linguistic ceremonies in the delivery room or other cultural practices. Lonesome George, the long-lived Galápagos giant tortoise , was male. He was not assigned male at birth — or rather, in George’s case, at hatching. A baby abandoned at birth may not have been assigned male or female by anyone, yet the baby still has a sex. Despite the confusion sown by some scholars, we can be confident that the sex binary is not a human invention.

Another downside of “assigned sex” is that it biases the conversation away from established biological facts and infuses it with a sociopolitical agenda, which only serves to intensify social and political divisions. We need shared language that can help us clearly state opinions and develop the best policies on medical, social and legal issues. That shared language is the starting point for mutual understanding and democratic deliberation, even if strong disagreement remains.

What can be done? The ascendance of “sex assigned at birth” is not an example of unhurried and organic linguistic change. As recently as 2012 The New York Times reported on the new fashion for gender-reveal parties, “during which expectant parents share the moment they discover their baby’s sex.” In the intervening decade, sex has gone from being “discovered” to “assigned” because so many authorities insisted on the new usage. In the face of organic change, resistance is usually futile. Fortunately, a trend that is imposed top-down is often easier to reverse.

Admittedly, no one individual, or even a small group, can turn the lumbering ship of English around. But if professional organizations change their style guides and glossaries, we can expect that their members will largely follow suit. And organizations in turn respond to lobbying from their members. Journalists, medical professionals, academics and others have the collective power to restore language that more faithfully reflects reality. We will have to wait for them to do that.

Meanwhile, we can each apply Strunk and White’s famous advice in “The Elements of Style” to “sex assigned at birth”: omit needless words.

Alex Byrne is a professor of philosophy at M.I.T. and the author of “Trouble With Gender: Sex Facts, Gender Fictions.” Carole K. Hooven is an evolutionary biologist, a nonresident senior fellow at the American Enterprise Institute, an associate in the Harvard psychology department, and the author of “T: The Story of Testosterone, the Hormone That Dominates and Divides Us.”

The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips . And here’s our email: [email protected] .

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