How colonial animals evolve

Abstract

The evolution of modular colonial animals such as reef corals and bryozoans is enigmatic because of the ability for modules to proliferate asexually as whole colonies reproduce sexually. This reproductive duality creates an evolutionary tension between modules and colonies because selection operates at both levels. To understand how this evolutionary conflict is resolved, we compared the evolutionary potential of module- and colony-level traits in two species of the bryozoan Stylopoma, grown and bred in a common garden experiment. We find quantitatively distinct differences in the evolutionary potential of modular and colony traits. Contrary to solitary organisms, individual traits are not heritable from mother to daughter modules, but colony traits are strongly heritable from parent to offspring colonies. Colony-level evolution therefore dominates because no evolutionary change can accumulate among its modules.

Fig. 1. A Stylopoma colony grows by adding modules and the population of Sylopoma colonies increase by the settling of sexually produced larvae.

Modules proliferate by asexual clonal budding. The growth of a single colony is illustrated in this series along with the numeric increase in the number of colonies. New colonies are founded from sexually derived larvae that disperse from a mother colony. The first sexually derived ancestral set of zooids is shown in the top left. Time runs from top to bottom so that by the end of this series, this colony has expanded markedly from the clonal proliferation of its modules, and the populations of colonies have also expanded from the sexual production of offspring colonies.

Fig. 2. We measure the evolutionary potential of these twelve traits.

Module-level traits are expressed by single modules. These traits are quantitative measures, lengths, widths, and densities. Colony-level traits are expressed by multiple modules and are measures of their relative positions, orientations, and numbers of modules. See the Supplementary Materials for a full description of these traits.

Fig. 3. The evolutionary potential for eight modular and four colony-level traits in two species of the bryozoan Stylopoma.

The parent-offspring phenotypic covariance for each trait is shown in columns. We calculate the heritability of member-level traits within each colony (indicated by gray circles jittered horizontally to avoid plotting overlap). The median heritability across colonies is shown by the large red circles. For colony-level traits, only a single estimate of heritability is possible; thus, the dispersion around the heritability estimate is indicated by vertical bars. Colony traits without bars do not vary among colonies.

Fig. 4. Map indicating the location of the Smithsonian San Blas field station in Panamá.