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Description
A central goal of evolutionary biology is to understand why diversification rates vary so widely among clades. Previous studies have suggested that variation in evolutionary rates of traits primarily under natural selection (for example, morphology, dispersal ability, or climatic niche) may drive speciation. Other studies have suggested that variation in evolutionary rates of traits primarily under sexual selection (for example, plumage, song, or behavior) may drive speciation. However, few studies have compared the effects of multiple traits on speciation or have quantified the relative importance of natural and sexual selection in shaping patterns of diversification. I used the tanagers (Thraupidae), the largest family of songbirds, to quantify the relative impacts of sexual and natural selection in speciation. I first used genomic approaches to infer phylogenies of the tanagers using concatenated and coalescent approaches. This phylogeny clarifies the position of enigmatic lineages within the tanagers and confirms the findings of earlier species-level phylogenies, providing a robust framework for hypothesis testing. The second objective of my thesis was to investigate links between plumage evolution and speciation rates. I found no evidence for the classic hypothesis that more complex color is related to speciation rates in the tanagers. Instead, high rates of plumage evolution are associated with higher speciation rates, regardless of whether species are evolving to become more colorful or more cryptic. The final objective of my thesis was to quantify the relative impacts of natural and sexual selection in speciation. I explored the relative contributions of four traits under natural and sexual selection (plumage, niche, song, and morphology) to speciation in the tanagers while also modeling evolutionary correlations between traits (for example, bill size constrains song frequency, plumage is shaped by habitat) and their effects on speciation rates. I found that the most variation in speciation rates can be explained by rates of song and plumage evolution, but that as a whole the best-fit model includes song, plumage, and niche. My results suggest that sexual selection can be a powerful force shaping speciation at macroevolutionary scales, but that speciation involves a complex interplay between traits under natural and sexual selection.
