During the Industrial Revolution in the 1800s, the smoke from burning coal in thousands of factories made the air so polluted that the tree trunks were coated with black soot.

Soon, the light-gray moth became conspicuous against the black tree bark, and birds ate them. A rare genetic variant arose that produced a black-colored moth, and soon only the black moths were common. Then when pollution laws came to Britain in the 1970s, the soot vanished from the air, the tree bark resumed its normal gray speckled color, and the speckled moths have returned, while the black variant is now rare again. This is one of the first and best demonstrations of natural selection ever documented.