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Animal populations can react to climate change by adapting to selection pressures, dispersing to new habitats, or undergoing plastic development at the individual level. These processes interact to form a net response, but it is difficult to determine the relative contributions of each process. In central and northern Michigan, two distinct mitochondrial lineages of Peromyscus leucopus (Upper Peninsula [UP] and Lower Peninsula [LP]) appear to have responded to climate change differently; one lineage (UP) has migrated into new geographic areas, while the other (LP) cannot. Six Michigan populations were sampled using genetic and geometric morphometric methods to test the effects of expansion versus nonexpansion demographic histories on genetic diversity and phenotypic variation. In pairwise population comparisons, there were no differences in genetic population structure or diversity between LP and UP populations or between older and newly established populations. However, the morphology of the cranium and especially the mandible were localized to each population, although moderate overlaps between the crania of UP populations and between the mandibles of Cheboygan (LP) and Menominee (UP) mice were observed. The results indicate that the UP lineage has rapidly dispersed into new areas without losing nuclear genetic diversity and that the morphological differences displayed among the populations are most likely due to plastic responses to local conditions.