THE EVOLUTION OF BRYOZOAN POLYMORPHISM IS ENABLED BY THE COMPETITIVE OPPORTUNITIES GIVEN BY A LIFE-HISTORY RATCHET

The major problem of social and colonial evolution resides in how colonial organisms make the transition from having mostly inherited member-level characteristics to having mostly novel colony-level characteristics. We don’t know how the tendencies and characteristics of the members end up channeled into a new roll within the context of the whole colony, nor do we know how new discrete variation arises from homogeneous and often clonal members. What we do know is that similar biological solutions to these two seemingly difficult problems have evolved independently multiple times in a wide range of organisms from bryozoans to bees. In this talk I propose a hypothetical mechanism for the evolution of polymorphism (division of labor), termed the "life-history ratchet" and explore its utility in understanding the evolution of cheilostome bryozoan polymorphism. This hypothesis explains the linkage between increases in polymorphism and reductions in the frequency of reproductive specialization through the repeated evolution of novel colony-level life-history strategies. With the origin of a novel polymorph type, novel life-history strategies become accessible which may be advantageous in the highly competitive encrusting environments where many bryozoans live. For example, primitive monomorphic species have a repertoire of life histories limited by the capabilities for asexual and sexual propagation of their members. After the origin of the initial reproductive polymorph, the life-history of the colony can vary independently of its members as a function of the relative frequencies of reproductive polymorphs in the colony. The life-history ratchet explains both how characteristics of solitary ancestors constrain the colony’s life-history and how reproductive specialization breaks this constraint by permitting new life-history strategies.