In the tropical islands in the South Pacific there is a population of blue moon butterflies. Some blue moon butterflies are larger than the others.
Changes in gene pool
The butterflies are struggling to survive because the predators such as the snake, and the two different type of birds that are trying to eat them. One of the butterflies is trying to fly away from the environment that their in because their trying to survive.
Only the butterfly that flies away faster and blend into the environment will survive from the environment that its in and pass those genes onto the next generation.
Since only the butterfly that flew away faster survived the predators because they blended in better with the environment, the genes pool in the population will produce better hidden butterflies.