Aggregate Trait Evolvability and Macroevolution in Two Sister Species of the Bryozoan Stylopoma

Abstract

The study of trait evolution in modular animals is more complicated than that in solitary animals, because a single genotype of a modular colony can express an enormous range of phenotypic variation. Furthermore, traits can occur either at the module level or at the colony level. However, it is unclear how the traits at the colony level evolve. We test whether colony-level aggregate traits, defined as the summary statistics of a phenotypic distribution, can evolve. To quantify this evolutionary potential, we use parent-offspring pairs in two sister species of the bryozoan Stylopoma, grown and bred in a common garden breeding experiment. We find that the medians of phenotypic distributions are evolvable between generations of colonies. We also find that the structure of this evolutionary potential differs between these two species. Ancestral species align more closely with the direction of species divergence than the descendent species. This result indicates that aggregate trait evolvability can itself evolve.

Levels of selection and macroevolution in organisms, colonies, and species

“…With expansion, Van Valen (1976, 1989, 1990 shows us another way to understand this shift that does not require colony members to be sterile or lose their reproductive abilities. With expansion, colony members can maintain reproductive abilities, but evolution can occur at the level of the colony, if the colony is the dominant level of heritability (Rice 2004;Simpson 2012;Simpson et al 2020;Leventhal et al 2023).…”

“…But the suppression of inheritance at the lower level also occurs. For example, in the colonial bryozoan Stylopoma, heritability at the lower part level is lost (Simpson et al 2020), while heritability at the level of the colony occurs (Simpson et al 2020;Leventhal et al 2023). With this, the evolutionary potential of the parts is neutralized, but their ability to increase in numbers remains.…”

“…With this, the evolutionary potential of the parts is neutralized, but their ability to increase in numbers remains. In the example of Stylopoma, the loss of heritability permits the growth of the colony by differential birth and death of zooids, and with the presence of heritability and fitness at the colony level (Simpson et al 2020;Leventhal et al 2023), evolution occurs at the level of the colony. Both the Stylopoma example and the thought experiment considering a case in colonies where growth is important for ecological (and therefore evolutionary) success highlight just how easy transitions to new levels of organization can be.…”