Thinking about evolution by taking development on board

What is evo-devo? Undoubtedly this is a shorthand for evolutionary developmental biology. There, however, agreement stops. Evo-devo has been regarded as either a new discipline within evolutionary biology or simply a new perspective upon it, a lively interdisciplinary field of studies, or even necessary complement to the standard (neo-Darwinian) theory of evolution, which is an obligate step towards an expanded New Synthesis. Whatever the exact nature of evo-devo, its core is a view of the process of evolution in which evolutionary change is the transformation of (developmental) processes rather than (genetic or phenotypic) patterns. Thus our original question could be more profitably rephrased as: What is evo-devo for? This section contributes many-faceted insights into the identity and scope of evo-devo.

According to Gerd Müller (Chapter 1), evo-devo is a discipline in its own right, because it asks a specific set of questions, solves biological problems that could not be solved by other approaches, and affects our understanding of evolutionary theory. After a short reflection on evo-devo history, the chapter examines in detail a set of evo-devo big questions. All these have at their core two interrelated components, namely how evolution affects development, and how the properties of developmental systems affect the course of evolution. Finally the author considers current evo-devo research programs, and discusses the impact of evo-devo on the theory of evolution.

Isaac Salazar-Ciudad (Chapter 2) critically reviews advantages and disadvantages of three 'schools of thought' in evolutionary biology that differ with respect to their views on the origin of variation: neo-Darwinism, the developmental constraints school and the developmental genetics school. He then presents a new set of concepts and studies that try to avoid the drawbacks of the three schools and argues that some aspects of the evolution of morphology and development are predictable if information is available about development and about the selective pressures that were operative in previous generations.

Wallace Arthur (Chapter 3) questions whether mega-evolution is more than just a result of the accumulation of micro/macro-evolutionary events, or, alternatively, if evolution is effectively a 'scale-independent' process. This question is approached by comparing magnitude, type and developmental timing of changes involved in high- and low-level divergence of lineages. He discusses three competing hypotheses: that mega-evolutionary changes are something quite apart from everyday changes; that mega-evolutionary divergences are statistically different from their lower-level counterparts; and that all levels of evolution are the same in both the absolute and the statistical sense.

Why do species show the patterns of diversity and disparity they do? Combining an exploration of how phenotypic variation is produced at each generation with an analysis of how this variation is influenced by natural selection and other extrinsic processes can provide the means for a comprehensive understanding of evolutionary patterns. Paul Brakefield (Chapter 4) presents a well-documented case study that illustrates an integrative approach linking the evolution of developmental mechanisms with the role of selection in the evolution of wing eyespots and other traits in Bicyclus butterflies.

Evo-devo aims to provide a mechanistic explanation of how developmental mechanisms have changed during evolution, and how these modifications are reflected in changes of organismal form. Thus, in contrast with studies on natural selection, which aim to explain the 'survival of the fittest', the main target of evo-devo is to determine the mechanisms behind the 'arrival of the fittest'. At the most basic level, the mechanistic question about the arrival of the fittest involves changes in the function of genes controlling developmental programs. Thus it is important to reflect on the nature of the elements and systems underlying inheritable developmental modification using an updated molecular background. Claudio Alonso dedicates a chapter (Chapter 5) to precisely this task.

In the search for evo-devo identity, Ronald Jenner (Chapter 6) starts from the perspective of an important, but neglected, epistemological dualism in a science like biology, that is, idiographics vs. nomothetics. Idiographics pertains to the description of unique and historically contingent particulars, while nomothetics pertains to the search for lawlike regularities or generalities. Thus, idiographically, evo-devo aims to document the unique effects of changes in evolutionary developmental mechanisms on the origin of novelties and the evolution of body plans. Nomothetically, it attempts to establish the general effects of evolutionary developmental mechanisms on determining the overall direction of phenotypic evolution. Recognising the dualism is not only conceptually important, but has also practical consequences, for example in the choice of model organisms.

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