Evolutionary genetics and Pleistocene biogeography of North American tree squirrels (Tamiasciurus)

Nucleotide sequence data from the mitochondrial DNA (mtDNA) cytochrome-b ge ne and allozymic data were used to infer the evolutionary and biogeographic histories of New World tree squirrels of the genus Tamiasciurus. Phylogene tic analyses of the cytochrome b data support the existence of 3 mtDNA line ages within Tamiasciurus: a western lineage consisting of populations of T. douglasii from western British Columbia (Canada), Washington, Oregon, and California, and T. mearnsi from northern Baja California (Mexico); a southw estern lineage consisting of populations of T. hudsonicus from New Mexico a nd Arizona; and a geographically widespread lineage comprising populations of T. hudsonicus from the remainder of the species' range. Levels of mtDNA sequence variation observed within and among populations of Tamiasciurus we re small (0-2.4%), suggesting that contemporary geographic patterns of gene tic variation in Tamiasciurus have been established relatively recently (i. e., in the Late Pleistocene). Allozyme analyses also support a close relati onship among extant populations of Tamiasciurus. No fixed allelic differenc es were observed among the 3 recognized species and interspecific genetic d istances (Nei's D) were substantially less than those typically observed be tween sibling species. Although differing from the current taxonomy in seve ral respects, geographic patterns of genetic variation observed within Tami asciurus are similar to those observed in a variety of North American borea l forest taxa and most likely reflect effects of forest fragmentation assoc iated with glacial cycles of the Pleistocene.