Principle #1 Genetic Variation: The Genetic Variation within the mice are the light colored and the dark colored mice.
Principle #2 Overproduction & Struggle for Survival: The overproduction is that the mother gives birth to twenty mice and only three survive. The dark colored mice will struggle to survive because due to the color of the fur it's noticeable for the predator.
Principle #3 Differential Survival & Reproduction: The light colored mice have a higher chance of surviving reproducing than the dark colored mice.
Principle #4 Change in the gene Pool: The advantageous of the light colored mice is that they will get more common in the population over time.