Answer:
One adaptation is white fur. There was probably one white cub born to a brown bear mother, and the fur helped camouflage it for safety and later, hunting. This bear would have been more successful in hunting in the arctic, so it was more likely to reproduce and pass on the white fur genes. A second adaptation is the long claws of brown bears that are used for digging through vegetation would have shortened to be safer on ice. Polar bear paws also have little sticky bumps on their paws for gripping ice.
A third adaptation is that polar bears don't hibernate in winter. In winter, there is a lot more food available to them, so they need to stay awake. A fourth adaptation is their hair is also now water repellent, because most of their food comes from the sea. A fifth adaptation is in their digestive system. Polar bears don’t digest carbohydrates as well as brown bears, because their diet has much more fat than anything else. Polar bears hunt seals and can convert the blubber into fat for their bodies to live off of when food is scarce.