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.

2Points
Population of Moth VarietiesGraph showing how the population of the peppered moth changed in a polluted versus unpolluted environment. In the unpolluted environment, the population size of the light variety of the moth was much higher than that of the dark variety. In the polluted environment, the opposite was true.
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.

Answer in at least 3 complete sentences.