THE EVOLVABILITY OF HEREDITY AND THE ORIGIN OF BRYOZOAN POLYMORPHISM

Polymorphism, or variation in body type, is a common feature of Cheilostome bryozoans, occurring in several independent evolutionary lineages. Bryozoan polymorphism is often compared to the caste system in ants, where heterochrony allows for different castes in a colony to occupy different morphological spaces. However, ontogeny in bryozoans operates differently than in ants. Individual zooids bud from their mothers, sometimes in several generations at once, and their exoskeletons are constructed separately. In this type of development, there is no ontogenetic progress from juvenile to adult morphology. Therefore, it is not viable to invoke heterochrony as an evolutionary determinant of polymorphism without empirical study. Here we show that the morphological differences between polymorphs and autozooids in several closely related species of the Cretaceous bryozoan Wilbertopora cannot be explained by heterochrony alone, and that a different evolutionary mechanism promoted the differentiation of zooid body type. We propose that the evolution of heredity between individuals in colonies allowed for polymorphism to evolve.