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Description
Increasing availability of species occurrence data, coupled with sophistication of niche modeling techniques, has made it possible to quantify species ecological niches based on large-scale environmental data for the first time. Using these data in conjunction with emerging, large-scale phylogenies of widespread groups has made it possible to address broad questions regarding the interplay between ecology and evolution. Tanagers are an ideal group for the study of the evolution of ecological niches. They are the largest group of Neotropical songbirds and are found in nearly all habitats across Central and South America. Thanks to the recent inference of a well-supported species-level phylogeny for the group, questions regarding their extensive ecological diversity can now be addressed appropriately in a phylogenetic framework. Specifically, questions regarding species climatic niches can be framed in an evolutionary context to examine the role of ecology in diversification. My first objective was to test whether differences in species numbers in the major clades of tanagers could be explained by differences in niche evolutionary rate among these major clades. This was accomplished by testing for correlations between rates of lineage diversification and rates of climatic niche evolution. I first tested whether or not constant-rate diversification was an appropriate descriptor of tanager lineage accumulation. As I found that tanagers are better described by density-dependent models of diversification, I used species richness and the carrying capacity K as descriptors of lineage diversity, as traditional measures of net diversification were not appropriate. I found that rates of niche evolution are correlated with K, which indicates that differences in species numbers can be explained by rates of niche evolution and thus niche evolution may be driving diversification in tanagers. I also found that tanagers with greater rates of niche evolution occupy greater environmental volume. My second objective was to examine niche evolution at a finer scale by examining niche characteristics among sister species pairs. There has been debate in the scientific literature in recent years regarding the role of ecology in speciation, and whether sister species should be expected to have similar or different niches. To address this, I took multiple approaches at several phylogenetic depths to examine niche evolution in tanagers. I measured niche overlap for 55 sister species pairs and found niche similarity to be low, implying niche divergence among sister species. I also calculated relative disparity through time and found that tanagers had partitioned available niche space early in evolutionary time, and that niche evolution eventually shifted toward convergence closer to the present. There was also significant variation in niche evolution dynamics across tanager subfamilies. The findings of my second chapter support the findings of my first chapter, in that both sets of results support the hypothesis that niche evolution might drive diversification in tanagers, and both suggest an important role for ecology in speciation in tanagers
