Color in peppered moths is controlled by genes. a light-colored variety and a dark-colored variety of a peppered moth species exist in nature. the moths often rest on tree trunks, and several different species of birds are predators of this moth.

before industrialization in england, the light-colored variety was much more abundant than the dark-colored variety and evidence indicates that many tree trunks at that time were covered with light-colored lichens.

later, industrialization developed and brought pollution which killed the lichens leaving the tree trunks covered with dark colored soot. the results of a study made in england are shown in the graph below.

state one possible reason that the light-colored variety was not completely eliminated from the polluted environment. be sure your reason can be supported by the evidence shown.