These remarkably adaptable bugs are often used as a textbook example of natural selection in action. Originally they evolved their "peppered" design to blend in perfectly when resting on light-colored trees and lichens. But due to excess pollution during the Industrial Revolution in England, many lichens died out and trees became blackened with soot. This made it easier for predators to find the moths, so the population began to evolve a darker, sooty coloration.
Today the lighter-colored moths are again commonplace, as environmental standards have improved.