Mar. 14, 2013 At the end of the last ice age, a population of
polar bears was stranded by the receding ice on a few islands in
southeastern Alaska. Male brown bears swam across to the islands
from the Alaskan mainland and mated with female polar bears,
eventually transforming the polar bear population into brown
bears. Evidence for this surprising scenario emerged from a new
genetic study of polar bears and brown bears led by researchers
at the University of California, Santa Cruz. The findings,
published March 14 in PLOS Genetics, upend prevailing ideas
about the evolutionary history of the two species, which are
closely related and known to produce fertile hybrids. Previous
studies suggested that past hybridization had resulted in all
polar bears having genes that came from brown bears. But the new
study indicates that episodes of gene flow between the two
species occurred only in isolated populations and did not affect
the larger polar bear population, which remains free of brown
bear genes. At the center