Here there is a population of mice living up in the rocky hills, and enough plantation to support them, with different predators. However, in these conditions, not many predators come around to eat them.
After a little while, the shrubbery/grass starts to die causing the mice population to drop overtime. Eventually, only those who were able to eat survive, even still they do struggle. Another result of this isthe predators either dying off, or migrating elsewhere to locate a better source of food.
During this time, the habitat stabilizes allowing the mice to repopulate. This time, in addition, acts as a grace period for everything else as the plants regrow, and predators return.
Durino this time there is a severe storm raining over the mountains causing a flood. This forces the mice to move upwards, and those who did not likely had a very low survival rate.
As many more predators venture back to the mountains, the white colored mice are more easily picked off, so they die more often especially with their disadvantage.
After all the chaos the mice went through, the strongest survived, this is an example of Charles Darwin's theory of natural selection.