The genetic structuring of populations can be influenced by present processes and past events. One of the largest historical events to affect the distribution and genetic characteristics of present-day North American biota is the Pleistocene glaciation. Thus, the study of post-glacial colonization patterns of species in northern landscapes can relay important ecological information, as species had to expand their range extensively following the retreat of the glaciers and are often at the terminal end of their expansion. These species consequently exhibit the genetic fingerprints of sequential founder events, in turn decreasing the genetic variation available for adaptation. Using amphibians to investigate post-glacial range expansion is advantageous, as they have limited dispersal abilities revealing fine-scale patterns and they are thought to be one of the first vertebrates to colonize post-glacial habitat. Therefore, to model the phylogeography of a primary colonizer and the population structure of anurans in northern landscapes, population genetics analyses of wood frogs (Lithobates sylvaticus) were performed in the James Bay area. Wood frogs were sampled from 17 localities around James Bay and genetic analyses were conducted with seven microsatellite loci and mitochondrial DNA sequences of the ND2/tRNATRP genes. Results show that the post-glacial recolonization of the James Bay area by wood frogs originated from the putative refugium in western Wisconsin, an area known as the Driftless Area. Two routes were taken by founders to colonize the James Bay area: one north-west of Lake Superior, colonizing western Ontario, and one through the Upper Peninsula of Michigan, colonizing southern and eastern Ontario and western Québec. Interestingly, the meeting of the two lineages south-west of James Bay led to the establishment of a zone of higher genetic variation than expected under the founder effect hypothesis. Additionally, population structure analyses revealed the segregation of three genetic populations east, north-west, and south-west of the bay, the latter showing the highest genetic variation and likely representing a zone of secondary contact. This study shows that past events such as post-glacial range expansions can explain present patterns of genetic variation and population structure, and that studies in northern landscapes may be very useful in understanding genetic patterns throughout the range of a species.

Sea levels fluctuated following glacial cycles during the Pleistocene, reaching approximately 115-130m below current sea levels in the Indian and Pacific Oceans during the last glacial maximum 17,000 years before present. The effects of these sea level fluctuations on population structure have been shown in many near-shore marine taxa, revealing several common patterns. However, the underlying mechanisms behind these observed patterns are largely unknown. Drops in sea level affect the distribution of shallow marine biota, exposing the continental shelf on a global scale, and displacing coral reef habitat to steep slopes where shelf breaks are shallow. In these circumstances, we expect that species inhabiting lagoons should show reduced genetic diversity relative to species inhabiting more stable outer reefs. Here, I tested this expectation on the scale of an entire ocean-basin with four wrasses (genus Halichoeres): H. claudia (N=194, with ocean-wide distribution) and H. ornatissimus (N=346, a Hawaiian endemic) inhabit seaward reef slopes, whereas H. trimaculatus (N=239) and H. margaritaceus (N= 118) inhabit lagoons and shallow habitats throughout the Pacific. Two mitochondrial markers (cytochrome oxidase I and control region) were sequenced to resolve population structure and history of each species. Haplotype and nucleotide diversity were similar among all four species. The outer reef species showed significantly less population structure, consistent with longer pelagic larval durations and a historically stable population. Mismatch distributions and significant negative Fu’s F values indicate Pleistocene population expansion for all species, and (contrary to expectations) reduced genetic diversity in the outer slope species. These data indicate that lagoonal species may persist through the loss of habitat, but are restricted to isolated refugia during lower sea level stands, which may inflate genetic diversity during high sea levels. Outer reef slope species on the other hand have homogeneous and well-connected populations through their entire ranges regardless of sea level fluctuations. These findings contradict the hypothesis that shallow species are less genetically diverse as a consequence of glacial cycles. / text

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AN ABSTRACT OF THE THESIS OF Elizabeth C. Geiger, for the Master's degree in Geology, presented on August 28, 2008, at Southern Illinois University Carbondale. TITLE: PALEOECOLOGY OF PLEISTOCENE GASTROPODS IN GLACIAL LAKE DEPOSITS IN SOUTHERN ILLINOIS/MISSOURI MAJOR PROFESSOR: Dr. Scott Ishman During the Pleistocene Epoch in the central Midwest United States multiple glaciations and interglaciations occurred in response to the advance and retreat of glaciers. This study focused on the Illinoian and Wisconsinan glaciations, which are separated by the Yarmouthian and Sangamonian interglacials. The advance of glaciers during glacial stages caused major rivers and their tributaries to aggrade creating slackwater lakes in many tributary valleys to the Mississippi River. Gastropod assemblages, found in glacial lake deposits, were used to interpret paleoecology during major glacial advances. Gastropods were collected from four sites in southern Illinois and Missouri to compare the Illinoian and Wisconsinan glaciations to one another, as well as to compare similar aged glacial lake deposits. This study identified forty-eight gastropod species, which were analyzed using statistical procedures. Hierarchical cluster analyses resulted in the identification of gastropod assemblages that were interpreted using habitat classifications. The results indicate specific similarities and differences between the Illinoian and Wisconsinan glaciations regarding environmental and climatic conditions.