If you simply looked at a polar bear and brown bear, you would probably never doubt that they are members of different species. Polar bears grow much larger than brown bears, and their paws have adapted to swimming long distances and to walking on snow and ice. Their white fur camouflages them, but the coats on brown bears are, well, brown - and their paws aren't adapted to water.

Clearly polar bears and brown bears are very different physically. But do physical characteristics tell the whole story? Remember the definition of species: "a group of similar organisms that can breed and produce fertile offspring." Well, polar bears and brown bears can mate and produce fertile offspring. They must be members of the same species, then. But are they? Look for clues in this chapter as to whether polar bears might be a separate species. Then, solve the mystery.