Polar bears evolved from the Brown(Grizzly) bear more than 200,000years ago. DNA taken from both types of bears is, for the most part, identical. As the Brown bear migrated north to the tundra and Arctic regions its physiology began to change. The ears and head got smaller and the fur turned white. The body elongated and formed a layer of insulating blubber(fat). The feet became webbed, and the bear's teeth evolved from long and sharp to shorter snd slightly rounded. All these changes allowed the Polar bear to withstand extreme cold, to move with grace and speed through the water and to chew and digest it's main prey, the seal. More carnivorous than any other bear, The Polar bear is an amazing result of evolution and is a true king of the north.
Information from "Adopt an Animal Newsletter, the Toronto zoo.