|I examined how variation within the yellow-throated warbler (Dendroica dominica) relates to geography. The yellow-throated warbler is a common neotropical migrant with a breeding range confined to the southeastern United States. It is divided into three continental subspecies that differ in ecology, morphology, and migratory behavior. In the first chapter, I used mitochondrial control region sequences to test whether eastern and western yellow-throated warbler subspecies showed temporal division consistent with a phylogeographic break, the Tombigbee River Discontinuity, that is known to have affected the phylogeographic patterns of several vertebrates in the southeast. Considerable genetic variation was uncovered in the yellow-throated warbler, but most of this variation was found within rather than between populations or subspecies. A shallow phylogenetic tree, star-like haplotype network, and unimodal mismatch
distribution all suggested a recent expansion. Coalescent modeling indicated that modern yellow-throated warbler populations are derived from a single common ancestral population and that differences between subspecies in morphology, ecology, and migratory pathways are the result of recent evolution. Some avian subspecies were described with insufficient evidence and do not reflect biological reality, so in the second chapter I performed a range-wide reassessment of the phenotypic differences between yellow-throated warbler subspecies to determine if there was a discrepancy between mtDNA and morphology. Results indicated much overlap in the morphological characters most important in diagnosing subspecies: bill length and proportion of yellow in lore, and discriminant function analysis failed to correctly assign most individuals especially those collected near the subspecies’ border. There was a strong west to east clinal change in bill length and proportion of yellow in lore and no evidence of discrete morphological groups. I recommend eliminating the subspecies D. d. albilora and D. d. stoddardi because they can not be reliably diagnosed by either morphology or mtDNA.