Multi-scale perspectives on the genetic structure of a widespread amphibian, the North American bullfrog (Rana catesbeiana)

Genetic variation is a conditio sine qua non for evolution, allowing populations to adapt to changing environments. This thesis explores questions at different spatial and temporal scales to determine the processes responsible for the development and maintenance of neutral genetic variation, and to explore its potential role in speciation, using the North American bullfrog (Rana catesbeiana) as the focal organism.; First is a phylogenetic and biogeographic analysis of the Rana catesbeiana species group---seven species of the family Ranidae endemic to eastern North America. Using cytochrome b and ND2 mitochondrial DNA sequence from multiple exemplars, I test the relative role of dispersal and vicariance in the evolution of this species group. Independent post-speciation dispersal events, likely from the Coastal Plain Region, best describe their diverse geographic distributions. Second, a comparative phylogeographic approach was used to explore the role of the Pleistocene in intraspecific diversification. The similar distributions of Rana catesbeiana and Pseudacris crucifer (Hylidae) conceal highly divergent phylogeographic structure based on homologous cytochrome b sequence data, with R. catesbeiana having lower intraspecific divergence and less geographic structuring. These data reflect either highly divergent rates of sequence evolution or, more likely, independent Pleistocene (and pre-Pleistocene) histories that may have been influenced by their divergent ecologies.; Third, I examine the previously discussed historical influences on the genetic structuring of northern breeding aggregations of R. catesbeiana . The scale at which gene flow can be confidently measured (controlling for history and other non-equilibrium conditions) is determined, and gene flow was estimated using seven highly polymorphic microsatellite loci. Localized gene flow (population pair-wise distances of 1 to 100 kms) was high, suggesting not only a prominent role for gene flow in the intraspecific structuring of R. catesbeiana, but also that breeding aggregations separated by up to 10's of kms do not adequately reflect distinct genetic populations in this species. Finally, I explore the hypothesis that the natural history and mating system of R. catesbeiana reflect adaptive female-biased dispersal in this species. My data show significantly greater female dispersal, and this in turn suggests that previous estimates of gene flow may be underestimated, given the sampling bias towards males in previous chapters.; By describing various levels of detail over which the genetic diversity of a species is structured, we obtain a better understanding of the relative importance of various microevolutionary processes in speciation. Together, the studies in this thesis reflect a prominent role for gene flow in the evolutionary history of, as well as contributing to the stability of contemporary 'populations' of R. catesbeiana.