Phylogeography and pleistocene evolution in the North American black bear.

To determine the extent of phylogeographic structuring in North American black bear (Ursus americanus) populations, we examined mitochondrial DNA sequences (n = 118) and restriction fragment length polymorphism profiles (n = 258) in individuals from 16 localities. Among the bears examined, 19 lineages falling into two highly divergent clades were identified. The clades differ at 5.0% of nucleotide positions, a distance consistent with an origin 1.8 MYA, and have different but overlapping geographical distributions. Areas of clade cooccurrence show that eastern and western populations are currently mixing, but regional differences in lineage distribution suggest that mixing has begun only recently. The long-term population history of black bears appears to be characterized predominantly by long-term regional isolation followed by recent contact and hybridization. Congruence between the pattern of diversity observed in black bears and patterns of forest refuge formation during the Pleistocene supports earlier speculation that Pleistocene forest fragmentations underlie a common pattern in the phylogeography of North American forest taxa.