At first the rabbits were all only dark brown for the spring and they camouflage in the dark forest from predators.
Then the rabbits produce offspring and one of them had a mutation and the mutated rabbit was white and the rest were dark brown.
Predators easily spotted the white rabbit and would try to change it in the dark forest. While the rest of the rabbits would camouflage and the predators would never see them.
Then an environment changed and the brown dark rabbits couldn't camouflage in the snow and the white mutated rabbit would easily camouflage cause of the white snow.
Predators easily spotted the brown rabbits and would catch the brown rabbits like it was nothing and the white rabbits could survive in the snow where they can camouflage.
Finally the white rabbits would produce offspring and they would produce so much white rabbits. The original dark brown rabbit alleles are harder to get and was quickly going away where the white rabbit alleles were more common produce.