Principle #1: Genetic Variation In the Blue Moon butterfly population, there are some female butterflies with a mutation for bacterial resistance against the Wolbachia bacteria. Other female butterflies do not have the mutation for resistance. The mutation can be inherited by the offspring.
Principle #2: Overpopulation & Selective Pressures The females who do not have the resistance mutation do not pass it to their offspring, so the males die. Females with the mutation pass it to their offspring, and the male offspring survive. In this case, the bacterial disease is the selective pressure.
Principle #3: Differential Survival & Reproduction Male butterflies who inherit the mutation from their mothers are resistant to the bacteria and survive. Because they survive, they live long enough to reproduce. When these mutated males reproduce, they pass on the resistance mutation to their offspring.
Principle #4: Changes in the Gene Pool Every generation will see an increase in the number of butterflies with the resistance mutation. On the other hand, every generation will have a smaller and smaller number of butterflies without the resistance mutation.