In the forest there is a population of peppered moths. Some moths are light gray and other dark gray.
Change in gene pool
The light gray version used to be more common because they blend into the environment but the environment change due to pollution, the light moth were no longer well camouflaged on the darkened tree trunks.
Predators were able to spot light moths and dark moths were more likely to survive and reproduce.
Since only the dark moth survived because they blended in the environment the population will produce all dark moths.