Thepolar bearis acarnivorousbear whose native range lies largely within theArctic Circle, encompassing theArctic Ocean, its surrounding seas and surrounding land masses. It is a large bear, approximately the same size as theomnivorousKodiak bear.A boar (adult male) weighs around 350–700kgwhile a sow (adult female) is about half that size. Although it is thesister speciesof thebrown bear,it has evolved to occupy a narrowerecological niche, with many body characteristics adapted for cold temperatures, for moving across snow, ice and open water, and for huntingseals, which make up most of its diet.Although most polar bears are born on land, they spend most of their time on the sea ice. Their scientific name means "maritimebear" and derives from this fact. Polar bears hunt their preferred food of seals from the edge ofsea ice, often living off fat reserves when no sea ice is present. Because of their dependence on the sea ice, polar bears are classified asmarine mammals;an alternative basis for classification as marine mammals is that they depend on the ocean as their main food source.

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Because of expectedhabitat losscaused byclimate change, the polar bear is classified as avulnerable species, and at least three of the nineteen polar bear subpopulations are currently in decline.For decades, large-scale hunting raised international concern for the future of the species, but populations rebounded after controls and quotas began to take effect.For thousands of years, the polar bear has been a key figure in the material, spiritual, and cultural life ofcircumpolar peoples, and polar bears remain important in their cultures.

The bearfamily,Ursidae, is thought to have split off from other carnivorans about 38 million years ago.TheUrsinaesubfamily originated approximately 4.2 million years ago.The oldest known polar bearfossilis a 130,000 to 110,000-year-old jaw bone, found onPrince Charles Forelandin 2004.Fossils show that between 10,000 and 20,000 years ago, the polar bear'smolar teethchanged significantly from those of the brown bear.Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in thePleistoceneor from the eastern part ofSiberia, (fromKamchatkaand the Kolym Peninsula).

The evidence from DNA analysis is more complex. Themitochondrial DNA(mtDNA) of the polar bear diverged from the brown bear,Ursus arctos, roughly 150,000 years ago.Further, somecladesof brown bear, as assessed by their mtDNA, are more closely related to polar bears than to other brown bears,meaning that the polar bear might not be considered a species under somespecies concepts.The mtDNA of extinct Irish brown bears is particularly close to polar bears.A comparison of thenuclear genomeof polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged approximately 603,000 years ago,although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar and brown bears, and establishes the divergence of polar and brown bears at 400,000 years ago.

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, theABC Islands bearshas between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating.Polar bears can breed with brown bears to produce fertilegrizzly–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 differentmorphology,metabolism, social and feeding behaviours, and otherphenotypiccharacteristics, the two bears are generally classified as separate species.

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When the polar bear was originally documented, twosubspecieswere identified:Ursus maritimus maritimusby Constantine J. Phipps in 1774, andUrsus maritimus marinusbyPeter Simon Pallasin 1776.This distinction has since been invalidated. One alleged fossil subspecies has been identified:Ursus maritimus tyrannusbecame extinct during the Pleistocene.U.m. tyrannuswas significantly larger than the living subspecies. However, recent reanalysis of the fossil suggests that it was actually a type of brown bear.