The Bering Land Bridge was the intermittent connection that allowed exchange of mammals between Asia and North America. Because some mammalian genera are widely distributed on both continents, recovery of phylogenetic histories of species within these genera may help reconstruct the sequence of intercontinental exchanges. We tested phylogenetic and biogeographic hypotheses in the widespread genus Microtus through parsimony and likelihood analysis of mtDNA-sequence data. The extant species of Microtus in North America are thought to be derived from multiple invasions from Asia or, alternatively, as a single invasion followed by autochthonous speciation. Mitochondrial cytochrome-b gene sequences were obtained for 78 individuals representing 24 species of Microtus. Data supported 1 clade of taiga voles (M. pennsylvanicus, M. montanus, M. townsendii, and M. canicaudus), a clade of Asian species (M. kikuchii, M. fortis, M. montebelli, and M. middendorffi), plus the Holarctic M. oeconomus and several other previously identified clades. M. gregalis also was found to be distant from M. abbreviatus and M. miurus, thus contradicting monophyly of the subgenus Stenocranius. Monophyly of North American species was supported, albeit weakly. Basal relationships were not robust, reflecting a single pulse of diversification about 1.3 × 106 years ago. This pulse mirrors the fossil record and may be partially responsible for the unstable taxonomic history.
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