<P> However, the two species have mated intermittently for all that time, most likely coming into contact with each other during warming periods, when polar bears were driven onto land and brown bears migrated northward . Most brown bears have about 2 percent genetic material from polar bears, but one population, the ABC Islands bears has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating . Polar bears can breed with brown bears to produce fertile grizzly--polar bear hybrids, rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar . However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears are generally classified as separate species . </P> <P> When the polar bear was originally documented, two subspecies were identified: Ursus maritimus maritimus by Constantine J. Phipps in 1774, and Ursus maritimus marinus by Peter Simon Pallas in 1776 . This distinction has since been invalidated . One alleged fossil subspecies has been identified: Ursus maritimus tyrannus became extinct during the Pleistocene . U.m. tyrannus was significantly larger than the living subspecies . However, recent reanalysis of the fossil suggests that it was actually a type of brown bear . </P> <P> The polar bear is found in the Arctic Circle and adjacent land masses as far south as Newfoundland . Due to the absence of human development in its remote habitat, it retains more of its original range than any other extant carnivore . While they are rare north of 88 °, there is evidence that they range all the way across the Arctic, and as far south as James Bay in Canada . Their southernmost range is near the boundary between the subarctic and humid continental climate zones . They can occasionally drift widely with the sea ice, and there have been anecdotal sightings as far south as Berlevåg on the Norwegian mainland and the Kuril Islands in the Sea of Okhotsk . It is difficult to estimate a global population of polar bears as much of the range has been poorly studied; however, biologists use a working estimate of about 20--25,000 or 22--31,000 polar bears worldwide . </P> <P> There are 19 generally recognized, discrete subpopulations, though polar bears are thought to exist only in low densities in the area of the Arctic Basin . The subpopulations display seasonal fidelity to particular areas, but DNA studies show that they are not reproductively isolated . The thirteen North American subpopulations range from the Beaufort Sea south to Hudson Bay and east to Baffin Bay in western Greenland and account for about 54% of the global population . </P>

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