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Hybridization and speciation in the Yellow-rumped Warbler complex Brelsford, Alan

Abstract

Hybrid zones provide insight into the speciation process. They show which characteristics have evolved differently between recently separated populations, and which differences promote reproductive isolation upon secondary contact. In the Yellow-rumped Warbler complex, the Myrtle and Audubon's forms hybridize in the Rocky Mountains. I investigated the history of the complex and the current evolutionary dynamics of the hybrid zone. In chapters two and three, I used genetic, plumage, and song analysis to show that the hybrid zone is stable and maintained by postmating barriers. Association between two genetic markers on different chromosomes showed that the hybrid zone is maintained by moderately strong selection against hybrids. Comparison between my sampling and data collected in 1965 showed that the hybrid zone remained stable during that time. Myrtle and Audubon's songs were weakly differentiated, with hybrid songs intermediate between them. Both parental types reacted as strongly to hybrid songs as to songs of their own type, and there was no evidence of assortative mating. In chapter four, I investigated the history of divergence and hybridization among the four Yellow-rumped Warbler subspecies. Previously, conflicting relationships had been hypothesized. Based on appearance, the Audubon's form had been grouped with two southern subspecies from Mexico and Guatemala. Another study of mitochondrial DNA had supported a very recent divergence between Audubon's and Myrtle and more distant relationship with the southern forms. Using nuclear markers, I showed that the Audubon's Warbler is closely related to the Mexican subspecies, but has acquired its mitochondrial DNA and a substantial fraction of its nuclear genome from the Myrtle form. This is the first multilocus genetic evidence for the hybrid origin of a widespread bird taxon. This thesis provides evidence for the diversity of mechanisms that may drive species divergence. Unlike many well-studied examples, neither premating barriers nor song maintain the distinctness of Myrtle and Audubon's Warblers. My work provides further evidence that hybridization can be a source of evolutionary novelty in animals, and highlights the importance of surveying multiple genetic markers in phylogeography. Finally, this study is a point of comparison for several other hybridizing species with similar biogeographic histories.

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