The mutation becomes a trait that can be transferred. Polar bears didn't exist until a mutation to a regular brown bear caused it's fur to be white. This white fur in a cold and white terrain gave it a hunting and camouflage advantage which caused it to thrive and survive. After mating, this trait was passed down and after generations of this, this trait became prominent. That's how it works. Same thing with all dogs, they all came from wolves that were domesticated and humans bred them based on their traits and behavior to create them for specific purposes. Over years if selective breeding, modern dog breeds are created. This is man made evolution where as the polar bear is natural.