There is a population of rabbits starts with only one type of allele (100%) , which means right now all of them have no differences, they are all the same.
Then suddenly, a mutation happens to the rabbit population, which causes one of the rabbits to change a color, and it looks different to the other rabbits, which is genetic variation.
The mice over produce so their offspring will get the allele that lets them blend in with dark environments, but since a mutation happened, the one rabbit that got mutated now stands out and attracts predators to eat it.
However, there can be an environment change so this will allow the mutated mouse to be better fit for the environment and camouflage. This will give the other dark color mouses a disadvantage . So, this will allow the mutated mouse reach the reproductive age.
So, because the original allele for the rabbits were dark colored, this now makes them spotted easily by predators, which makes them weak.
Now, since the mice with the original alleles are now slowly dying or fading away, now the mice that are mutated has a better chance in surviving the population because now they blend in .