According to Darwinism, evolution occurs because, in populations, there is heritable phenotypic variation, and then ecological factors differentially affect the contribution of each of those variants to the next generation. Thus, to understand the way in which phenotypes in a population change over generations (this is the direction of evolutionary change) two questions need to be addressed: (1) which phenoty-pic variants arise in each generation, and (2) which of these variants are filtered out by ecological factors in each generation. In each generation, and assuming no dramatic genomic rearrangements, developmental dynamics determine which morphological variation arises from genetic and environmental variation. Developmental dynamics are currently not very well understood and thus the question of which phenotypic variants arise in each generation is not well understood either. A different emphasis is given to each of these two questions by different approaches or schools of thought in evolutionary biology.
For many evolutionary biologists, especially those close to the core of neo-Darwinism, this lack of understanding about development has not always been perceived as a limit on progress in understanding morphological evolution (Haldane 1932, Mayr 1982). For some authors (Haldane 1932) this does not derive from lack of understanding development. Instead, natural selection is seen as the main or unique force determining how the phenotype changes (Fisher 1930,
Evolving Pathways: Key Themes in Evolutionary Developmental Biology, ed. Alessandro Minelli and Giuseppe Fusco. Published by Cambridge University Press. # Cambridge University Press 2008.
Charlesworth et al. 1982). The question of which phenotypic variation arises and what its role is in determining how phenotypic distributions change over time is either not addressed or assumed to be unanswerable. Which variation is produced is essentially seen, according to the neo-Darwinian view, as unpredictable, other than that it should be abundant, small (gradual) and, in principle, possible in any trait.
These views about the role of development and variation in evolutionary theory have found many detractors during the past century, especially since the 1980s (Alberch 1980, Goodwin 1994, Newman and Müller 2000). This research claims that development has an important effect in evolution because it produces variation that differs from the variation assumed by the neo-Darwinian view. These studies introduce concepts such as developmental constraint, developmental bias, evolutionary novelty and robustness (Müller, Chapter 1 of this book) to describe aspects of evolution that are not conveniently approached from the Modern Synthesis. Often these approaches are not perceived as an integrated or consensus alternative to the Modern Synthesis nor as being in total agreement with it. Most of this research, which will be referred to here as the constraint school, involves conceptual (Alberch 1980, Horder 1989, Goodwin 1994, Fusco 2001, Arthur 2004) and experimental embryology studies (Richardson and Chipman 2003, Newman and Müller 2005) considering how development can affect morphological evolution, but not (with some exceptions: Newman and Müller 2000) how developmental mechanisms themselves evolve.
The advances in developmental genetics towards the end of the twentieth century have been perceived (Gilbert and Sarkar 2000) as a unique opportunity to integrate development into evolutionary theory. Many of these studies are concentrated on the search for the ultimate molecular differences underlying morphological differences between species. This does not directly explain which morphological variation is possible by genetic variation in development (question 1). Most of the research in developmental genetics is experimentally driven. The extraction of general theoretical insights to explain which morphological variation development can produce is just at its beginning, and it is still not clear how these insights will modify evolutionary theory or the relationship between the developmental constraint school and the Modern Synthesis.
This chapter critically reviews some advantages and disadvantages of these three schools of thought, neo-Darwinism, the developmental constraints school and developmental genetics, in understanding the way in which morphology changes in evolution (as described). It will be discussed how some assumptions of neo-Darwinism contrast with what is currently known about morphological variation and its developmental bases, how the constraint school and developmental genetics share or require some neo-Darwinian assumptions, and how research agendas in developmental genetics can be slightly rephrased so that they can be more easily used in the other two schools and, in general, for the study of how development affects evolution. A new set of concepts and studies that try to avoid these disadvantages will be explained. These will be used to argue that some aspects of the evolution of morphology and development are predictable if some information is available about development and selective pressures in previous generations. These new concepts allow us to approach questions that are not within reach for these three schools. Inferences will be made about the relative involvement of different types of developmental mechanisms in the evolution of morphology under different selective pressures. In general the aim is to explain how the study of pattern formation can help in understanding which morphological variation is possible in development (question 1) and, at the same time, how development itself evolves.
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