In this population of beetles, some of them are green and others are a tan color.
The green beetles are more visible to predators than the tan beetles. Therefore, their chance of survival and reproducing are smaller. The black birds are easily spotted by prey instead of the light blue birds.
Since the tan beetles are more fit than the green ones, they started becoming more common.
The gene population changed from being mostly green to mostly tan due to surviving longer and creating more offspring.