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Description
Studying macroevolutionary patterns of phenotypic variation and its driving forces in large radiations is crucial to understanding how biodiversity is generated across broad spatiotemporal scales. I used the tanagers (Thraupidae), the largest family of songbirds, to explore patterns of morphological evolution and its correlates to selection and speciation. I first investigated how multiple aspects of morphology interact to impact song evolution. I found that bill size is more tightly associated with frequency-related song parameters than with body size, contrary to past studies. My results suggest that within the tanagers bill morphology may be more strongly associated with song variation, perhaps due to biomechanical constraints. The second objective of my study was to investigate convergent evolution. As a first step, I used phylogenetic regressions across the tanager phylogeny to explore correlations between diet guild, climate, and bill morphology. After finding a significant association between bill shape and diet guild, I quantified the extent of bill convergence in response to ecological selection. I found that most tanagers showed phenotypic convergence, with multiple groups exhibiting significant convergence relative to phylogenetic distance. In addition, some species and clades were found to occupy unique morphospace. My study shows that across Thraupidae, various bill shapes have evolved convergently to fill multiple distinct sections of ecological niche space, likely in response to ecological opportunity. The third objective of my study was to explore the relationship between rates of speciation and morphological evolution within Thraupidae. I found significant positive correlations between rates of evolution in all traits and speciation rates across the phylogeny. I also found that clades within certain subfamilies (Coerebinae, Sporophilinae, Poospizinae) display rates of trait evolution that noticeably differ from the background rate of the tree. Shifts in phenotypic evolutionary rate predominantly occurred within traits related to bill dimensions. These shifts occurred most consistently on branches leading to the Darwin’s finches, supporting past hypotheses positing this clade’s exceptional diversification has transpired under ecological selection. Shifts identified within Sporophilinae suggest that morphological evolution may also play a role in diversification in this group.
