Graduate Thesis Or Dissertation
 

Natural history and evolution of a color polymorphism in Rana pipiens, the northern leopard frog

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  • A primary goal of population genetics is to identify the role of microevolutionary forces in producing observed patterns of molecular and phenotypic variation. I conducted four studies in the northern leopard frog, Rana pipiens, to determine just how mutation, migration, genetic drift, and selection influenced, genetic structure of mitochondrial DNA (mtDNA), nuclear DNA, and a single locus polymorphism that determines dorsal coloration. In the first study, I surveyed the literature concerning color and pattern polymorphisms in anurans. I conclude that anuran polymorphisms remain a rich but largely unexploited system for studying the evolution of phenotypic variation in nature. In the second study, I compared mitochondrial DNA variation from 35 populations distributed across the species' range. A phylogenetic analysis indicated R. pipiens is split into two deeply divergent mtDNA groups, a western group and an eastern group. Phylogeographic and demographic analyses indicated that although restricted gene flow with isolation by distance explained the majority of the processes influencing current genetic structure, population bottlenecks and expansions also played an important role. In the third study, I investigated mtDNA and microsatellite variation in Pacific Northwest populations of R. pipiens, where a recent range contraction had occurred. I found that peripheral populations had reduced levels of genetic variation compared to more interior populations. Moreover, I found that historic samples from peripheral population already had reduced levels of genetic variation. Therefore, low diversity in the remnant populations could not be ascribed to the recent range contraction. In the fourth study, I compared genetic structure from a suite of putatively neutral molecular markers with that derived from the color polymorphism locus. Genetic structure at the color locus, assessed both spatially and temporally, was indistinguishable from structure at neutral loci. This study exemplifies the importance of investigating for evidence of selective maintenance before studies attempt to measure the selective mechanisms maintaining a polymorphism. Overall, my research helps to elucidate how biogeographic and microevolutionary forces influence a wide-spread North American species, R. pipiens.
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