Molecular Biogeography in the Pocket Mice (Perognathus and Chaetodipus) and Grasshopper Mice (Onychomys): the Late Cenozoic Development of a North American Aridlands Rodent Guild

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Abstract
Paleoecological and geological evidence previously have been used to infer a sequential development of arid landscapes in western North America composed of: a gradual Miocene development of savanna and semidesert habitats; a rapid late-Miocene expansion of regional deserts, grasslands, and shrubsteppes; cyclical late Pliocene-Pleistocene changes in the distribution and composition of aridlands biotas. Rodents that characterize aridlands regions in western North America include lineages detectable in the fossil record back to the late Oligocene as well as mid-Miocene immigrants from Asia. In this study, nucleotide-sequence data from the mitochondrial DNA genes COIII and cytochrome b were used to analyze phylogeny, genetic divergence, and geographic structure within the pocket mice (Perognathus and Chaetodipus) and grasshopper mice (Onychomys). Molecular phylogenies, estimates of divergence times, and geographic distributions suggest that middle to late Pleistocene glacial cycles appear to have had little influence on lineage divergence in aridlands rodents. Rather, a correspondence between molecular, distributional, paleoecological, and geological evidence suggests that divergence and current distributions of most extant lineages can be attributed to late Tertiary-early Quaternary development of the western North American cordillera. These data are used to produce a predictive generalized model of late Cenozoic area relationships among arid regions in western North America.

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