Roger Sansom and Robert N. Brandon (eds.): Integrating Evolution and Development: From Theory to Practice
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- Reydon, T.A.C. Acta Biotheor (2011) 59: 81. doi:10.1007/s10441-010-9121-x
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One might compare organismal evolution and organismal development to an old couple, of which the two partners had drifted apart over the years, experienced a profound crisis in which they didn’t talk to one another, but in the end have found their way back to a happy relationship.
For the longest time, the study of life was not compartmentalized into the separate and occasionally competing fields of investigation that we have today, but took place as part of a larger endeavor of studying the natural world. Both ‘evolution’ and ‘development’ featured in this endeavor as general terms denoting change in the living domain and the terms were used largely interchangeably. (The Latin ancestor of the word ‘evolution’ means ‘to unfold’ or ‘to develop’ in the sense of unfolding an intrinsically present potential; see, e.g., Bowler 2003: 8.) In the nineteenth century, however, the two terms began to drift apart as each acquired a technical meaning in the context of the development of biology as a separate domain of science. ‘Development’ came to denote the development of individual organisms from seed or fertilized egg to adult. And ‘evolution’ now was what happens to populations of organisms, not individual organisms, as they change in time as a consequence of old generations dying off and new generations being born.
Throughout much of the twentieth century, the study of evolution was the domain where the action was and in which the major advances were made: from the rediscovery of Mendel’s laws of heredity in 1900, via the establishment of population genetics, the forging of the Modern Synthesis in the 1930s–1940s, the discovery of the molecular structure of DNA (which, of course, was a discovery in molecular genetics, but fell into place as clarifying one missing element in evolutionary theory, namely the mechanism of inheritance), up to Dobzhansky’s famous dictum that “nothing in biology makes sense except in the light of evolution” (1964: 449, 1973). The study of development was pushed into the background during this time and took place largely disconnected from the study of evolution. And, as the editors of the present volume write, this was no accident, no case of one domain of investigation just happening to move forward at a much more rapid pace than another domain. Rather, the separation of the study of evolution and the study of development was (at least partly) intended: “The conceptual separation of development and biology [sic!] was widely seen as an important step in the rapid advance of biology in the twentieth century, because it allowed evolution to be studied without getting bogged down in the messy details of development.” (p. vii—unless otherwise indicated, page references are to the book under review).
In the past decades, biologists have come to realize that this thought is no longer appropriate. Perhaps the rapid advances in biology in the past century were indeed made possible by decoupling the study of evolution from the study of development (I suppose this topic should make for an interesting study in the history of biology), but today the opposite is true: rather than being bogged down by them, the study of evolution cannot proceed without looking closely at the messy details of organismal development. The result of this realization is the emergence of evolutionary-developmental biology—or “evo-devo” for short—as one of the newest branches on the tree of biological science (for a popular introduction, see Carroll 2005). It is, however, not entirely clear what evo-devo is. Some describe it as a new interdisciplinary field of work (p. 18), a specialization, a discipline (p. 195), or an interfield (p. 59) in the sense of Darden and Maull (1977) within biological science. But equally legitimately it can be thought of as a research program (pp. 20, 195). The diversity of approaches and lines of work subsumed under the common denominator of ‘evo-devo’ is, it seems, too large to think of evo-devo as a homogeneous discipline or research program (pp. 35–40). But conceived of as a “developmental synthesis” (pp. viii, 3), evo-devo comes down to an attempt to integrate developmental biology into the Modern Synthesis, in much the same way as the Modern Synthesis integrated evolutionary theory, population genetics, systematic biology and paleontology (pp. 26–30). As such, evo-devo can be thought of as one step in the ongoing further elaboration of the synthetic theory of evolution (e,g., Kutschera and Niklas 2004; Jablonka and Lamb 2005; Müller 2007; Pigliucci 2007; Rose and Oakley 2007; Padian 2008; Pigliucci 2009; Pigliucci and Müller 2010).
Integrating Evolution and Development is an edited collection of seven articles that provides a sample of the main issues in evo-devo, sketches the history of the “field” and addresses some of the philosophical issues that the new developments in evo-devo give rise to. By being edited by two philosophers of biology and written by a team of contributors made up of philosophers of biology and theoretical (or at least theoretically inclined) biologists, this volume constitutes a nice example of how close (and overlapping) philosophy of science and theoretical science can—and should—be.
Chapters 1 and 2 provide a meta-perspective on evo-devo. In Chapter 1, theoretical biologist Manfred D. Laubichler and historian/philosopher Jane Maienschein discuss the history of work on embryology and organismal development in the nineteenth and twentieth centuries to clarify in what respects evo-devo is a new line of thought and in what respects it continues the tradition that already existed. According to Laubichler and Maienschein, the turn of the nineteenth to the twentieth century witnessed a crucial break from a situation in which development, inheritance and evolution were studied within a single theoretical framework to a new situation in which these topics came to lie at the focal points of separate lines of investigation. And, Laubichler and Maienschein show, this state of affairs has become so entrenched that it is hard to transform into a state in which research becomes more integrated. Accordingly, evo-devo, they point out, is not established yet but is still very much in the making. And the eventual success of this process will depend on whether a new set of concepts will be found on which the new approach to biology can be founded.
Chapter 2, a 68-page work by philosopher Werner Callebaut and biologists Gerd B. Müller and Stuart A. Newman, present a philosophical analysis of the nature of evo-devo and what they call the “organismic systems approach” to development and evolution. Callebaut and co-authors begin their discussion by adding to the historical sketch of the preceding chapter, showing how evo-devo emerged as a response to a widespread view that the Modern Synthesis was essentially incomplete. They go on to explicate in detail the central concepts, methods and explanatory strategies of evo-devo, as well as the various ways of doing evo-devo research, showing that ‘evo-devo’ is the name for a collection of quite diverse approaches and lines of investigation. One important distinction here is between 2 “subagendas” of evo-devo, namely evo-devo (that causally prioritizes evolution over development) and devo-evo (that endorses the opposite prioritization). A bit slogan-like, evo-devo-ists hold that “evolution has resulted in organisms that develop”, while devo-evo-ists would say that “development has resulted in populations of organisms that evolve” (p. 44). The organismic systems approach that the authors put forward as their preferred approach to—or, research program for?—the study of evolution stands in the line of the latter subagenda.
The remaining five chapters of the book turn to some of the methodological and conceptual details of evo-devo research and present new ideas aimed at contributing to the further development of the developmental synthesis. In Chapter 3, biologist H. Frederik Nijhout addresses a central problem for attempts to achieve clarity about how development and evolution hang together, namely the problem of modeling how genotypes relate to phenotypes. Researchers have long tried to understand the ways in which organisms’ phenotypes depend on their genotypes by constructing genotype-phenotype maps, i.e., mathematical models in which phenotypes are modeled as mathematical functions of combinations of genes. Nijhout remarks that these models typically are linear models that model individual genes as acting additively or multiplicatively in producing a phenotype. But the actual relation between genotypes and phenotypes is more complex and essentially depends on the cellular context, he points out, such that any realistic genotype-phenotype map should take the complex nature and environmental dependency of gene interaction into account. This is something that simple, linear models fail to do. In this chapter, Nijhout develops a non-linear model for constructing genotype-phenotype maps that is intended to describe the complex reality of gene action more closely that the commonly used linear models.
In Chapter 4 (another long one of 60 printed pages), biologist Gerhard Schlosser discusses the role of constraints in the production of organismal form. Selection and constraints are often seen as opposing factors in explanations of organismal form. The traditional debate is between those who emphasize the importance of selection in evolution, arguing that environmental demands guide evolutionary change, and those who emphasize the importance of structural constraints in evolution, arguing that evolutionary change is highly restricted by what kinds of structures are developmentally possible in the first place (pp. 113–115). Schlosser argues that the debate between these two positions is misguided: the question is not which of the two factors is the principal one in explanations of organismal form. Schlosser gives a philosophical analysis of the concept of constraint as well as a scientific discussion of how constraints on evolution may arise to argue that constraints and selection should be thought of as complementary, not rival, factors. Any good explanation of organismal form and evolutionary change should refer to both.
Chapter 5, by philosopher Roger Sansom, looks at the notion of evolvability, the “the ability of random mutations to sometimes produce improvement” (p. 173) or, in an alternative formulation, “intrinsic potential of certain lineages to change during the course of evolution” (p. 31). Whether one conceives of evolvability in terms of the potential for improvement through evolution or merely the potential for evolutionary change, on both views important questions are which factors contribute to evolvability, what makes one structure more evolvable than another and how evolvability can arise as a product of organismal development and evolution. (In the latter case, the “evolution of evolvability” is under consideration.) Sansom argues that explanations of evolvability need to invoke a transgenerational unit of selection: “evolvability can only be selected for across generations, because evolvability is only realized across generations” (p. 186; my emphasis). The underlying idea is that evolvability is a potential of a lineage, not of an individual organism, as evolvability is linked to the accuracy of heredity: the more accurate heredity is, the less severe the impact of mutation (as mutations tend to occur one by one, not with many in the same reproductive step), so the better the chance for a mutation to remain in the population and be selected. Sansom introduces the notion of legacy to denote such multi-generation units of selection. In asexual organisms, a legacy is an individual plus its line of ancestors; in sexually reproducing organisms, a legacy is an individual plus its parents, one pair of grandparents, one pair of great-grandparents, and so on back in time (although it is an open question how far back in time; pp. 187–188). Evolvability, on this account, is to be explained by the selection of legacies.
Chapters 6 and 7 extend the scope of evo-devo by looking at the borderline between biology on the one hand and psychology and anthropology on the other. In Chapter 6, philosopher Paul E. Griffiths follows up on the near-lying thought that “If the ideas that make up evo-devo have been so productive in opening up new lines of investigation into morphological evolution, they may be equally productive for psychological evolution” (p. 196). Griffiths focuses on the notions of modularity and homology as they occur in the conceptual framework of evo-devo and explores whether—and argues that—these notions can play important roles in explaining the origin of psychological traits. Rather than further building on the conceptual framework of evo-devo, Griffiths thus extends the scope of this framework—work that I think can be interpreted as showing that evo-devo should be thought of as a loosely interconnected research program or a step in biological theorizing, rather than as a field or discipline that has come to be added to the spectrum of the biological sciences (see above).
The final chapter of the volume (Chapter 7, the longest one counting 97 pages and written by philosophers William C. Wimsatt and James R. Griesemer), is on evo-devo studies of the cultural realm. Wimsatt and Griesemer argue that cultural development (i.e., the acquisition of culture by growing-up humans) needs to be studied in detail in order to be able to understand how cultural evolution works. Accordingly, the authors develop an evo-devo theory of culture that hinges on the notion of scaffolding. Growing-up humans develop within the cultural context that is provided by their environment, which in turn can be thought of as constituting a sort of supporting structure—a scaffold—for the development of the human being. But, Wimsatt and Griesemer argue by means of theoretical arguments as well as detailed case studies, this scaffold is not a passive, static structure. Rather, the cultural scaffold should be understood as an interactive, two-way system in which the scaffold supports the individual’s cultural development while the individual’s cultural development causes small changes to the scaffold. (Wimsatt and Griesemer compare the situation to Neurath’s boat.) Because of this interaction between cultural evolution (the product of which is the scaffold that supports development) and cultural development, an evo-devo approach to understanding how cultural change occurs is required.
Besides their remark that all chapters in the book “argue for the significance of evo-devo” (p. ix), the editors don’t say much about the aims of the volume or their target audience. I have, therefore, looked at the book from the perspective of the two most common usages of scientific books: as an introductory/overview text and as a novel contribution to the field. Integrating Evolution and Development might serve as an introductory text to evo-devo for advanced readers, as it provides a broad view of the domain—without aiming for completeness, though—and in addition presents novel work in an accessible manner. Chapters 1 and 2 give the background that is necessary to understand why today there is such a thing as evo-devo in the first place and provide a thorough analysis of what, exactly, evo-devo is or could/should be. The biology-oriented Chapters 2–5 go into some of the scientific and philosophical details and present new contributions to biological theory. Chapters 6 and 7 extend this conceptual work by considering issues on the boundary between biology and psychology and the boundary between biology and psychology/anthropology, respectively, and presenting new contributions to theorizing on these issues. The book thus would be an introduction to evo-devo the hard way, challenging its readers while simultaneously giving them the basic ideas, and as such could serve as a graduate-level classroom or reading group text for students specializing in philosophy of biology, or in biology with a heavy emphasis on theory. But I think the book succeeds better as a collection of cutting-edge papers that introduce new ideas that will advance evo-devo—and as such it is essential reading material for professional biologists as well as philosophers of biology.