BOLKER, J.A.; University of New Hampshire: Evolving modularity: evolutionary origins and consequences of modular developmental resources

Evolution of both phenotypes and genotypes occurs through changes in the form and interrelationships of biological modules. Developmental modules have themselves evolved, and continue to do so. Modular developmental resources (Gass and Bolker, 2002) can incorporate both genetic and environmental information, and are subject to selection based on their contribution to the final organismal phenotype. I suggest that selection favoring modularity of developmental resources has contributed both to the initial evolution of multicellular organisms with complex ontogenies, and to their subsequent phenotypic elaboration. The different ways developmental resources are drawn upon in extant lineages may influence the continuing evolution of those modules, as well as of the lineages that comprise them. For example, the deployment of a single developmental resource within individuals may be one-time (as with certain morphogenetic modules specific to early stages of animal development) or repeated (e.g. the continued elaboration of meristem-derived structures in individual plants); the latter situation allows selection to act more directly on the environmental responsiveness of a given module. Another evolving aspect of modularity is the extent of coupling of developmental resources within an individual, particularly compared to the dependence of those modules on external environmental signals. Where internal connectivity is strong and external responsiveness weak, developmental canalization and potentially reduced evolutionary flexibility may result. In contrast, the ability to draw upon internal developmental resources independently (and repeatedly) and to integrate environmental signals into patterns of resource deployment may be a source of evolutionary flexibility and phenotypic novelty.