Allozymic patterns among populations of Ochotona princeps indicate a pre-Wisconsin (>120,000 years ago) divergence of four major genetic units in the western United States (northern Rocky Mountains, southern Rocky Mountains, Cascade Range, and Sierra Nevada). Pikas initially spread south along cordilleran dispersal corridors during a pre-Wisconsin glacial stage, and were fragmented into isolated montane refugia during a subsequent interglacial. Range re-expansion during the Wisconsin allowed secondary contact of genetic units, while montane glaciers, pluvial lakes, and glacial lakes limited secondary contact in some areas. Low genetic variation in all examined populations of O. princeps is due to: the metapopulation structure of pikas, dictated by the fragmented nature of a narrowly defined habitat and maintained by low vagility and high philopatry; accelerated loss in small or isolated habitat patches; accelerated loss in summer-drought regions; and, in the Cascade Range genetic unit, derivation of current populations from a genetically depau-perate source population.
