Principle #1 Genetic Variation The darker butterflies have become immune to predators in their environment. The light brown butterflies are not immune to the predators because they do not blend in to their surroundings. This mutation can not be inherited. This has lead to the light brown butterflies to decrease their population.
Principle #2 Overproduction and the Struggle for Existence The females without mutation the male offspring will die. The females with the mutation the male offspring will live. They struggle to survive if thye have a disease (bacteria).
Principle #3 Differential Survival and Reproduction The males that do inherit the resistance mutation will survive. When they reproduce they pass on the mutation to the offspring.
Principle #4 Changes in the Gene Pool: The male population with the mutation will increase. The males that do not have the mutation their population will decrease.