Butterfly's of the yellow and blue variety overpopulate an area and start to dominate the forest.
Natural Selection
Over time, a new type of colored butterfly is born, and it blends in more with the environment.
Predators Keep Hunting
Due to living in a forest, there are predators. The predators are able to find the blue and yellow butterfly easier than the brown ones and the yellow and blue butterfly population drops.
Breeding
Soon the population starts to change into the brown butterflies, and the amount of yellow and blue butterflies go down.
Soon most of the butterflies that don't blend into the environment continue to get eaten.
Soon most of the population turns into brown butterflies.
