There are two polar bears, there's one with a thick coat of fur, and the other has a very thin coat of fur. Their environment is extremely cold and now that it's winter, it's only getting colder.
When giving birth, the one's with a thick fur of coat always will have children that'll have a higher survival rate. Then the thin coated ones will lose their offspring's easier due to the high temperatures in their environment.
As time goes, the polar bear;s with very thin coats of fur will die off because of their lack of warmth. And the thick ones will get to reproduce and have them with a higher chance of surviving the environment.
The drastic change in the gene pool is that the thick polar bears had the higher fitness, making them the one's to successfully have children and grow, unlike the thin ones that could die out easily.