In the sandy deserts, there lived two different groups of insects. Due to natural genetic variation, One group of insects were green and the other group was tan.
On a typical day, A bird is passing by looking for food until he spots the green insect. He dives and swoop up his food as the tan insect blends in to his surroundings and scurries away.
As days pass by in the sandy desert, there becomes less and less green insects. On the other hand, the populations of the tan insects increases as their survival rate is greater than that of the green insects.
Over time, the sandy habitats changes into a grass-filled meadow. This new environment now will help the green insects blend in better whereas it's the opposite for the brown insects.
Now more and more tan insects are getting eaten whereas green insects are being less eaten. The green insects are better adapted to their surroundings and therefore they tend to survive and produce more offsprings.
With a higher survival rate, the green insect's population is able to increase whereas the tan insect's population decreased. Being more environmentally fitter than the brown insects, the green insects will have more offspring that are able to have this favorable variation. By being green, it will help them survive and blend in to their environment.