EvoDevo Shapes the Extended Synthesis

Abstract

When the interfield of EvoDevo arose in the early 1980s, its momentum was fueled by both the methodological advances in molecular biology and the explanatory deficits of the standard evolutionary paradigm, the Modern Synthesis. Whereas the methodological progress led to a proliferation of empirical research, the implications of EvoDevo for evolutionary theory often went unnoticed or were actively sidelined by the stakeholders of the received theory. Only with the progressive corrosion of the Synthesis framework due to challenging evidence from several more fields (genomics, epigenetics, physiology, behavior, etc.) the tide started to change, and EvoDevo has become one of the key players in the current redefinition of evolutionary theory. The evidence produced by EvoDevo mostly bears on the complexity of the genotype–phenotype rapport and the gene regulatory changes underlying its evolutionary transformation. It also elucidates the cell and tissue properties involved in the generation of complex structures, the physics and physiology governing these processes, as well as the quantitative assessment and modeling of generative procedures in evolutionary contexts. These results have tremendously improved our understanding of how development originated in the context of multicellularity, how its repertoires evolved, and how organismal change is mechanistically realized. EvoDevo has been equally prolific in the conceptual domain, contributing a wealth of new principles to the evolutionary model. Besides elaborations of such classical issues as heterochrony and developmental constraint, EvoDevo gave rise to new concepts such as facilitated variation, developmental modularity, morphoregulation, epigenetic innovation, developmental systems drift—to name but a few. These provided improved understanding of the evolvability of developmental systems and their contribution to evolutionary robustness and non-gradual phenomena of phenotypic change. The result of these endeavors was a broadened interpretation of the role of development in the evolution of organismal complexity. Today it can be concluded with much confidence that the generation of selectable variation and phenotypic structure is not merely a consequence of genetic variation but also of the capacity of plastic developmental systems to respond to natural selection and direct environmental induction with integrated and often emergent reactions. Development not only constrains and facilitates the generation of phenotypic variation, but also affects the tempo, mode, and directionality of evolutionary change. In concert with the evidence emerging from other fields of evolutionary biology, as briefly mentioned above, the findings of EvoDevo mean that nearly all assumptions central to the traditional Modern Synthesis framework have been overturned: evolution is not just a variational, continuous, and incremental kind of change but involves alternating speeds and discontinuous events; not all organismal features are independently adaptive, but certain traits are emergent and evolutionarily neutral; phenotypic variation is not merely a product of genetic variation that arises randomly and at constant rates but is equally a product of integrated development and plasticity; no single Gerd Muller is president of the European Society for Evolutionary Developmental Biology.