Although the two species are relatively closely related, grizzly bears and polar bears differ in terms of their fur, dentition, behavior and natural habitat. Most of the key differences between the two species relate to their differences in habitat and lifestyle.
Grizzly bears are black, brown, gold or reddish-brown, while polar bears are almost completely white. Both rely on camouflage to sneak up on their prey undetected. Polar bears have thicker layers of fat and fur than grizzly bears do, which helps them to keep warm in the icy polar waters that they frequent.
The foot pads of polar bears are much larger than those of grizzlies, which allows the sea-dwelling bears to swim very well. The teeth of the two species reflect the difference in habitat preference; grizzly molars are adapted for mastication of plant material, while those of polar bears are well suited for chewing animal flesh.
Researchers think the grizzly bear might have evolved from the polar bear about 100,000 years ago. While polar bears and grizzly bears have mated and produced viable young in captivity, the two species do not cohabitate in the wild, so natural hybrids are exceedingly rare. The first documented evidence of natural hybridization was collected in 2006, when hunters shot a strange-looking bear. A DNA test confirmed that the animal was indeed a hybrid of the two species of bear, according to NBC News, quoted by Wikipedia.