Phylogeography of a post-glacialcolonizer: Microtus longicaudus (Rodentia : Muridae)

The molecular phylogeography of Microtus longicaudus was investigated with DNA sequences of the mitochondrial cytochrome b gene. We used phylogenetic and pairwise distance methods to reconstruct the history of the species wit h particular emphasis on the Pacific Northwest. Genetic variation across th e species was consistent with vicariant events during the Pleistocene and s ubsequent northern postglacial expansion following the receding Laurentide and Cordilleran ice sheets. The largest break (> 6% uncorrected sequence di vergence) was found to exist between populations found southeast of the Col orado River (eastern Arizona, Colorado, Wyoming and New Mexico) and all oth er western populations. Other well-supported subclades were composed of sam ples from: (i) the islands and north coast of southeast Alaska; (ii) easter n Alaska, British Columbia, Washington and Oregon; and (iii) northern Calif ornia, Idaho and Montana. Within subclades, divergence was low. Our results suggest that the close relationships among haplotypes within northern subc lades are a result of recent colonization, whereas higher among-subclade di vergence is caused by genetic differentiation during prolonged periods of i solation, possibly as a result of mid-Pleistocene climatic events.