EVOLVABILITY AND MACROEVOLUTION OF COLONY-LEVEL TRAITS IN TWO SPECIES OF BRYOZOANS

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. But, in a group of animals called 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 evolvable across a generation of colonies in two sister species of the extant bryozoan Stylopoma, grown and bred in a common garden breeding experiment. We find that components of phenotypic distributions, specifically the median and median absolute deviation of trait values of colony members, are evolvable between generations of colonies. Furthermore, this evolvability has macroevolutionary importance because it correlates with the morphological distance between these two species. Since these phenotypic distributions are evolvable, and this evolvability corresponds to evolutionary divergence between species, we infer that these distributions can shift and change shape over macroevolutionary timescales. Such changes to phenotypic distributions across many generations of colonies may underpin the emergence of colony-level traits, like division of labor in colonies.