Division of labor and recurrent evolution of polymorphisms in a group of colonial animals

Abstract

Rendering developmental and ecological processes into macroevolutionary events and trends has proved to be a difficult undertaking, not least because processes and outcomes occur at different scales. Here we attempt to integrate comparative analyses that bear on this problem, drawing from a system that has seldom been used in this way: the co-occurrence of alternate phenotypes within genetic individuals, and repeated evolution of distinct categories of these phenotypes. In cheilostome bryozoans, zooid polymorphs (avicularia) and some skeletal structures (several frontal shield types and brood chambers) that evolved from polymorphs have arisen convergently at different times in evolutionary history, apparently reflecting evolvability inherent in modular organization of their colonial bodies. We suggest that division of labor evident in the morphology and functional capacity of polymorphs and other structural modules likely evolved, at least in part, in response to the persistent, diffuse selective influence of predation by small motile invertebrate epibionts. © 2011 Springer Science+Business Media B.V.