The brown bears of coastal Alaska have been recently regarded as comprising from one to three distinct genetic groups. We sampled brown bears from each of the regions for which hypotheses of genetic uniqueness have been made, including the bears of the Kodiak Archipelago and the bears of Admiralty, Baranof and Chichagof (ABC) Islands in southeast Alaska. These samples were analysed with a suite of nuclear microsatellite markers. The 'big brown bears' of coastal Alaska were found to be part of the continuous continental distribution of brown bears, and not genetically isolated from the physically smaller 'grizzly bears' of the interior. By contrast, Kodiak brown bears appear to have experienced little or no genetic exchange with continental populations in recent generations. The bears of the ABC Islands, which have previously been shown to undergo little or no female-mediated gene flow with mainland populations, were found not to be genetically isolated from mainland bears. The data from the four insular populations indicate that female and male dispersal can be reduced or eliminated by water barriers of 2-4 km and 7 km in width, respectively.

Mitochondrial DNA was purified from five American species of geese representing the genera Anser and Branta, which have fossil records. The results of electrophoretic comparisons of about 75 fragments per individual produced by 14 restriction enzymes imply that the mean extent of sequence divergence between species of Anser and Branta is about 9%. Fossil evidence suggests that these two groups of geese had a common ancestor 4-5 million years ago. Thus, the mean rate of sequence divergence in goose mitochondrial DNA is not far from 2% per million years, the value in mammals.

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Genetic and paleontological evidence are combining to provide a new and surprising picture of mammalian biogeography in southeastern Alaska. Prior to our study, the brown and black bears of the Alexander Archipelago were considered postglacial immigrants that never had overlapping ranges. Vertebrate fossils from caves on Prince of Wales Island now demonstrate that brown and black bears coexisted there (and even inhabited the same caves) both before and after the last glaciation. Differences in mtDNA sequences suggest that living brown bears of the Alexander Archipelago comprise a distinct clade and are more closely related to polar bears than to their mainland conspecifics. We conclude that brown bears, and perhaps other large mammals, have continuously inhabited the archipelago for at least 40,000 yr and that habitable refugia were therefore available throughout the last glaciation.