UC Davis

Coloniality, in which organisms are composed of repeated discrete structural units that are physically connected to each other, is a major evolutionary innovation. Since the first colonial animals appeared during the Cambrian Period, coloniality has independently evolved in several animal lineages including cnidarians, hemichordates, chordates, bryozoans, entoprocts, and annelids. Despite the prevalence of this trait, the mechanisms underlying broadscale biogeographic trends in coloniality remain untested. In this dissertation, I examined two global-scale biogeographic patterns related to coloniality in ascidians, a diverse group of chordates. In addition to this work, I also examined colonial animals through the lens of pedagogy. In Chapter 1, I investigated mechanisms underlying the latitudinal gradient in colonial ascidian richness. Using publicly available species occurrence data and abundance data from two large experimental networks, I demonstrate that this gradient is robust and likely driven by ecological mechanisms. In Chapter 2, I explored the relationship between coloniality and Rapoport’s rules, the correlation between latitude and species’ latitudinal or bathymetric ranges. Using species occurrence data, I show that ascidians demonstrate Rapoport’s bathymetric rule possibly due to biogeographic variation in coloniality. Together, these findings indicate the need for more rigorous testing of the mechanisms underlying broad biogeographic patterns. In Chapter 3, I examined taxonomic bias in the animal diversity present in introductory biology course materials. Vertebrates, especially mammals, were overrepresented in textbooks and lecture slides, relative to their actual diversity. In contrast, invertebrates, including many groups of colonial animals, were underrepresented to varying degrees.