Dissecting the geographical expansion of Peromyscus leucopus in the northern Great Lakes: Insights from genetics, morphometrics and ecology

Geographical and ecological distributions of biological species are primarily limited by prevailing climate conditions, and thus it is hardly surprising that recent rapid climate change has triggered evolutionary processes that are deeply affecting the biology of many organisms. One consequence of rapid climate change has been the occurrence of shifts in geographical ranges of many species, such as the white-footed mouse (Peromyscus leucopus), which has experienced a northern range expansion in Michigan and Wisconsin over the last few decades and as a consequence is now common in localities where it was absent until recently. In this study, I sought to understand some of the historical causes and consequences of the expansion process undergone by this rodent species. I analyzed complete D-loop sequences from 595 mice collected throughout the Great Lakes region to investigate the origin of the newly established populations. Phylogeographic analyses strongly support two lineages, spanning populations from Wisconsin through the western Upper Peninsula (UP), and from the Lower Peninsula (LP) to the eastern Upper Peninsula (UP) of Michigan, respectively. With few exceptions, western populations in the UP originate from Wisconsin while the eastern UP populations originate from LP. Both lineages display a genetic signature of spatial expansion, whereas demographical expansion is apparent only for the western UP-Wisconsin lineage. I also investigated whether the rapid geographic expansion of populations of this species in the northern Great Lakes region has been accompanied by rapid morphological divergence, and whether the pattern of divergence suggests a neutral process or reflects the influence of non-random factors. Using morphometric analyses of mandible shape, I found significant differences between recently expanded and long established populations not only in the magnitude of shape variation but also in the dimensionality of variation, which suggest that the expansion process has been accompanied by rapid morphological change along directions of the phenotype space that are absent in ancestral populations. Furthermore, phenotypic and phylogeographic patterns are incongruent in these populations, suggesting that changes in morphology have not been caused by random phenotypic drift. To further explore possible causes for the phenotypic divergence observed between old and recent populations, I examined the association between geographical and environmental variables and patterns of genetic and morphometric variation. Results from these analyses suggest that even though most factors analyzed contribute to the global variation in the mandible, cold temperatures and snowfall magnitudes appear to be the best predictors for the differentiation between old and new populations.