There are two types of rabbits in the forest; these are natural variation in the population.
In winter it snows in the forest and snow covered much of the land.
The brown and white rabbits are preyed upon by the eagles. the brown rabbits are easily spotted by the eagle from the sky.
The animal with the best chance of survival is the white rabbits during winter, since it blends in with the snow covered background.
The brown rabbits are hunted and less of them are observed in the population. Leaving the population of rabbits predominantly white coloured
The white fur have trait have a high adaptive value during winter and allow white rabbits to survive and breed more viable offspring. This increases the frequency of the white fur allele in the gene pool - evolution has occurred.