Aggregate trait evolvability and macroevolution in two sister species of the bryozoanStylopoma

Abstract

The evolution of trait variation among populations of animals is difficult to study due to the many overlapping genetic and environmental influences that control phenotypic expression. In a group of animals, bryozoans, it is possible to isolate genetic contributions to phenotypic variation, due to the modular nature of bryozoan colonies. Each bryozoan colony represents a snapshot of the phenotypes that correspond to a single genotype, which can be summarized as a phenotypic distribution. We test whether these phenotypic distributions are heritable across generations of colonies in two sister species of the bryozoan Stylopoma, grown and bred in a common garden breeding experiment. We find that components of phenotypic distributions, specifically median trait values of colony members, are heritable between generations of colonies. Furthermore, this heredity has macroevolutionary importance because it correlates with the morphological distance between these two species. Because parts of phenotypic distributions are heritable, and this heritability corresponds to evolutionary divergence between species, we infer that these distributions have the potential to evolve. The evolutionary potential of these phenotypic distributions may underpin the emergence of colony-level traits, like division of labor in colonies.