| Climate changes during the Quaternary have had dramatic effects on the distributions of organisms and communities in the Mojave and Sonoran deserts of North America. Herein, I evaluate the phylogeography of two amphibian species inhabiting these regions that have undergone substantial population declines in recent decades, the relict leopard frog, Rana onca (= Lithobates onca) and the lowland leopard frog, R. yavapaiensis (= L. yavapaiensis). This thesis comprises two chapters. In the first chapter, I summarize literature describing taxonomy, phylogenetics, and conservation status for these two species. I then explore the possible effects of Quaternary climate changes on distribution of these frogs within regions. I also provide examples of phylogeographic patterns from other generally co-distributed organisms to develop background for interpreting the phylogeographic structure of R. onca and R. yavapaiensis that I describe in Chapter 2. This chapter investigates the phylogeography of R. onca and R. yavapaiensis using sequence data from mitochondrial DNA (mtDNA) to assess 276 individuals representing 30 sites from across current distributions. Chapter 2 represents an article (Olah-Hemmings et al. 2010) in press (Journal of Zoology), co-authored with Jef R. Jaeger, Michael J. Sredl, Martin A. Schlaepfer, Randy D. Jennings, Charles A. Drost, David F. Bradford, and Brett R. Riddle. Therefore, I use plural pronouns throughout this chapter to be consistent with the article. The analysis supported a previously determined phylogenetic break between taxa, with no admixing of R. onca and R. yavapaiensis haplotypes within sites. The phylogeographic assessment, however, further divided R. yavapaiensis into two distinct mtDNA lineages, one representing populations across Arizona and northern Mexico and the other a newly discovered population within the western Grand Canyon (Arizona). Estimates of sequence evolution indicate a possible Early Pleistocene divergence of R. onca and R. yavapaiensis, followed by a Middle Pleistocene separation of the western Grand Canyon population of R. yavapaiensis from the main R. yavapaiensis clade. Phylogeographic and demographic analyses indicate population or range expansion for R. yavapaiensis within its core distribution that appears to predate the latest glacial maximum. Species distribution models under current and latest glacial climatic conditions suggest that R. onca and R. yavapaiensis may not have greatly shifted ranges since the last glacial maximum. |
