Phylogeography of the dusky shrew, Sorex monticolus (Insectivora, Soricidae): insight into deep and shallow history in northwestern North America

Phylogenetic relationships among the dusky shrew (Sorex monticolus) and eig ht related species (S. bairdi, S. bendirii, S. neomexicanus, S. ornatus, S. pacificus, S. palustris, S. sonomae and S. vagrans) were assessed using se quences from the mitochondrial cytochrome b gene (801 bp). Analyses using p arsimony and maximum likelihood revealed significant molecular variation no t reflected in previous morphological studies of these species. Conversely, three morphologically defined species (S. bairdi, S. neomexicanus and S. p acificus) were poorly differentiated. Sorex ornatus and S. vagrans represen ted basal taxa for a more inclusive group that included: (i) a widespread C ontinental clade containing S. monticolus (Arizona to Alaska, including S. neomexicanus); (ii) a Coastal clade containing S. monticolus (Oregon to sou th-east Alaska, including S. bairdi and S. pacificus); (iii) the semiaquati c species (S. bendirii and S. palustris); and (iv) S. sonomae. Additional s ubdivision was observed within the Continental clade corresponding to popul ations from the northern and southern Rocky Mountains. Average uncorrected sequence divergence between the Coastal and Continental clades was 5.3% (ra nge 4.5-6.2%), which exceeds many interspecific comparisons within this spe cies complex and within the genus Sorex. Lack of resolution of internal nod es within topologies suggests a deep history of rapid diversification withi n this group. Late Pleistocene/Holocene glacial perturbations are reflected in the shallow phylogeographic structure within these clades in western No rth America. Our results suggest also that S. monticolus is not monophyleti c under current taxonomic nomenclature. This perspective on phylogeographic history was developed within a growing comparative framework for other org anisms in western North America.