Principal #1 Genetic Variation: A mutation caused some mice to make their fur turn black. There is now genetic variation in the population of mice, white mice, and black mice.
Principal #2 Overproduction & the Struggle for Existence The white mice have about 50 babies a year, but most are eaten because the white mice have the inability to camouflage from predators.
Principle #3 Differential Survival & Reproduction The black mice are have a higher fitness because they can camouflage in the rock, avoiding predators. This allows them to survive long enough to reproduce more than the white mice.
Principle #4 Changes in Gene Pool Overtime, the black fur trait in the mice becomes more common in the population because it gives them the higher advantage over the white mice. As the white mice are eaten, the black mice reproduce, making the trait more common.