In the Arctic there are 2 types of bears. White large bears and tan smaller bears.
Changes in the Gene pool
Every so often there is a snowstorm. The small tan bears struggle so hide and survive because their coats are thinner and don't blend in to hide from other bears. 2 tan bears freeze to death.
The white bears survive the storm and pass on their genes to the next generation.
The white bears lived and reproduced and since the only tan bear died of old age there are now only white bears. Their adaptations were advantageous for this environment and from now on there will only be white bears here.