In the snow forest there is a population of deer. Some deer are white and some are grey.
Changes In Gene pool
The grey deer are struggling to survive due to the overpopulation so since they are slow and exposed in sight they are easily targeted and chased over so that they will be hunted by wolves or bears or other vicious predators out there in the wilderness. As in for the white deers are more likely to survive because they are probably more likely to not be targeted.
Only the white deer will survive in this environment and it will pass on its offspring so that the rest will have a higher chance of surviving
Since only the white deers survived the predators because they blended in with the environment. The gene pool in the population will produce more white deers.