THE EVOLUTION OF ALLOMETRY OF MODULES IN CHEILOSTOME BRYOZOANS

Allometry, the relationship between the size of an organism and other aspects of its morphology, is a means of studying constraints and selection. Most allometric studies focus on solitary organisms and extant populations. Unchanging allometric relationships indicate evolutionary constraints while differing allometric relationships among populations or species suggest selection. However, the results of allometric studies are difficult to generalize due to multiple sources of heterogeneity and sampling error. Here, we alleviate some of the typical challenges in allometric studies by using colonial organisms as a model system. Rather than studying changes in trait size versus body size, we use repeated measurements of module size of the same genetic individual, to capture variation due to environmental plasticity. We study allometric relationships between module size and three different traits (namely a reproductive trait – brood chamber size; a resource capture trait – orifice size; and a defense trait – avicularia size) over 2 million years of evolution of the cheilostome bryozoan Microporella in New Zealand. Specifically, we ask if the slopes and intercepts of narrow-sense allometry of log trait size versus log module size change over time and among species. We find that in general, on the 10⁴ to 10⁵ year timescale, in species lineages, the mean-centered intercepts of these allometric relationships can change dramatically, but not their slopes. However, both intercept and slope can be different among even closely related species. The constraints among different traits are nevertheless different and their population variances inform us also on the different evolvabilities of these traits. Using our results, we speculate on any difference in the evolution of allometric relationships among colonial versus solitary organisms and discuss why colonial organisms are an especially good model for studying constraints and selection on different timescales.