Modularity in animal development and evolution: Elements of a conceptual framework for EvoDevo

For at least a century biologists have been talking, mostly in a black-box sense, about developmental mechanisms. Only recently have biologists succee ded broadly in fishing out the contents of these black boxes. Unfortunately the view from inside the black box is almost as obscure as that from witho ut, and developmental biologists increasingly confront the need to synthesi ze known facts about developmental phenomena into mechanistic descriptions of complex systems. To evolutionary biologists, the emerging understanding of developmental mechanisms is an opportunity to understand the origins of variation not just in the selective milieu but also in the variability of t he developmental process, the substrate for morphological change. Ultimatel y, evolutionary developmental biology (EvoDevo) expects to articulate how t he diversity of organic form results from adaptive variation in development . This ambition demands a shift in the way biologists describe causality, a nd the central problem of EvoDevo is to understand how the architecture of development confers evolvability. We argue in this essay that it makes litt le sense to think of this question in terms of individual gene function or isolated morphometrics, but rather in terms of higher-order modules such as gene networks and homologous characters. We outline the conceptual challen ges raised by this shift in perspective, then present a selection of case s tudies we believe to be paradigmatic for how biologists think about modular ity in development and evolution. (C) 1999 Wiley-Liss, Inc.