Natural selection leads to evolutionary change when individuals with certain characteristics have a greater survival.Also it could be that reproductive rate than other individuals in a population and pass on these inheritable genetic characteristics to their offspring.
The polar bear stays warm with the surroundings of the tundra since it has thick fur.This increases the fitness of the polar bears since it keep them warm since they wont die of coldness.
This species of polar bears shows genetic variation type of fur.Because of a mutation, the polar bears were able to change their thin fur coats to a thick fur coat.
More polar bears that can survive are reproduced.In addition to finding food and space, those are the best able to avoid dying of coldness so they have higher chances of surviving.
The thick fur coat polar bears allows them to stay warm.They will be able to survive and be the ones to reproduce.
As generation pass more and more thick fur coat polar bears are surviving and reproducing.This allows thick fur coat polar bears to increase in the polar bear population.