Evolutionary genetics and phylogeography of tassel-eared squirrels (Sciurus aberti)

The tassel-eared squirrel (Sciurus aberti) is confined to a montane archipelago of mixed conifer forests in the American Southwest. Explanations for its fragmented distribution vary, invoking historical scenarios that stress late-Pleistocene vicariance, dispersal, or some combination thereof. To test among these hypotheses, we examined the phylogeography of S. aberti, as determined by variation in mtDNA restriction sites among 22 populations sampled range-wide. Genetic distance and maximum-parsimony analyses revealed three distinct lineages of mtDNA, which in turn form two major phylogeographic assemblages; eastern (Mexico; New Mexico-Colorado-Utah) and western (Arizona-southwestern New Mexico). Comparisons between the maximum-parsimony tree and an area cladogram of regional mountain ranges showed marked topological discordance, dismissing late-Pleistocene vicariance as a principal phylogeographic factor. Certain mtDNA data support the post-Pleistocene dispersal hypothesis, in which squirrels dispersed northward in conjunction with the rapid range expansion of ponderosa pines (Pinus ponderosa). However, divergence levels observed between assemblages (1.8%) also suggest a significant, underlying vicariant event responsible for major separation of lineages during the early Pleistocene. Thus, the evolutionary genetics of S. aberti has been influenced profoundly, and perhaps repeatedly, by climatic change during the Pleistocene.