A long time ago there was two types of bears that were always competing for food.
Bears are reproducing and their population is growing causing a lack of food.
As more and more bears are being born the competition for food is increasing causing bears to die. Especially while bears.
White fur and short claws
Brown fur and long claws
The white bears are dying making the brown fur and long claws genes more important for survival. Long claws and brown fur is being inherited and increases chance of survival.
Now most offspring have brown fur and long claws that helps them because they can use their claws to eat fish and their fur to sneak up on other animals. Because of natural selection as time passes the bears will evolve to have long claws and brown fur.
As time passes, generation after generation, bears will evolve to have brown fur and long claws due to the advantage they provide. There will be no to very few white bears.