There are two different fur color of rabbits in the population: tan/red and white.
Overproduction and Struggle to Struggle
Tan/red rabbits are struggling to survive because they are easily spotted by foxes. White rabbits blend in with the environment allowing them survive longer and reproduce more offspring.
Differential Survival and Reproduction
The tan/red rabbits are not fitted to the snowy environment. The white rabbits are able to reproduce and pass on the white fur trait to the next generation.
Changes in Gene Pool
The white rabbits have the beneficial trait of white fur allowing them to camouflage in their snowy environment away from predators. Since they are the only ones surviving, the gene pool will increase and become all white rabbits.