You are studying moth coloration for a population in a temperate forest. At the beginning of your study, most moths were a light brown color. After monitoring the population for a few years, a predator was introduced into the forest, and within a few years, you see that the population is predominantly a dark brown color that closely matches the bark of the trees. The predator is then removed from the forest, and within a few years, the moths population is mostly light brown. What phenomenon best describes what is occurring in the moth population