The Origins of Biodiversity Patterns in Vertebrates: New Insights Into Old Questions

Abstract

Species richness varies greatly among habitats, geography, and groups of organisms. What factors are responsible for these differences, and how did these differences arise over time? Within a clade, species are added through speciation and removed through extinction. Within a region or habitat, species can also be added through colonization. Organismal traits or environmental factors may influence richness indirectly by affecting speciation, extinction, or colonization. The goal of this dissertation is to investigate how these three processes have varied over time to form the species richness patterns observed today. To do this, I take advantage of four recent developments in evolutionary biology: (1) the collection of DNA sequence data across many taxa; (2) the time-calibration of molecular phylogenies; (3) the increased complexity of models of diversification and ancestral reconstruction; and (4) the aggregation of taxonomic, geographic, paleontological and trait data into public databases. In Appendix A, we investigate the disparity in amniote species richness between marine and nonmarine habitats. We found that, surprisingly, there was no systematic difference in rates associated with habitats. However, there was a strong relationship between species richness and the timing of habitat transition. Many marine transitions went extinct, and as a consequence almost all living marine lineages are Cenozoic in age. In contrast, amniotes have occupied land uninterrupted for over 300 million years. We concluded that extinction and time interact to produce the richness disparity between marine and nonmarine habitats. In Appendix B, we investigated the causes of the peak in marine richness at the Central Indo-Pacific (CIP) hotspot using percomorph fishes as a focal clade. We found that diversification rates were similar among warm oceans, and highest in cold oceans. The high diversity of the CIP is due to many lineages that colonized from 34–5.3 million years ago and then diversified in-situ. Other oceans have fewer colonizing lineages and/or more recent colonization leaving limited time for in-situ diversification. In Appendix C, we investigated the causes of the latitudinal diversity gradient on continents, using freshwater ray-finned fishes as a model clade. Colonization time explained 2–5 times more of the variation in species richness than diversification rates among 3,000+ freshwater drainage basins. The Neotropics is species-rich because it has supported steady diversification for ~100 million years, while other tropical regions have had periods of low diversification. While high-latitude lineages that are dominant today did not arrive until the Cenozoic, they diversified at a comparable or higher rate than tropical lineages. Finally, in Appendix D we tested the longstanding hypothesis that sexual dichromatism increases diversification rates. We tested this hypothesis at three phylogenetic scales: across all ray-finned fishes, within individual clades, and within nested subclades of the largest clades. We found no difference in rates between monochromatic and dichromatic fishes at the scale of all fishes. Only a few clades showed a relationship with dichromatism and diversification. Surprisingly, these clades did not include the subjects of classic population-level studies (e.g. cichlids). Support for such a relationship increased in most smaller subclades examined. We concluded that when sexual dichromatism influences diversification, its effects will be localized to small phylogenetic scales. Overall, the works in this dissertation yield new insights into longstanding problems in evolution and ecology.