There are two variations of moth: the light-colored and the dark-colored.
The moths are preyed on by birds. Because the dark moths stand out against the light bark, they are targeted.
However, because of a nearby factory, the bark begins to darken in color. This changes which moths are now easily visible as the darker moths will be able to blend in better than the light moths.
The light-colored moths are now the ones being primarily preyed on by birds which causes number of = light-colored moth to decrease.
Due to the decrease in light-colored moths, the populatoin is mostly dark-colored moths.
In the beginning, the light-colored moths had a higher survival rate because of the light bark. However, this changed as did the color of the bark as the darker color allowed the dark-colored moths to blend in and made the light-colored moths an easier target for prey.