Peppered Moth

A Peppered Moth (Biston betularia)was in the moth trap yesterday morning - another new species for the year.

This species provides a good example of evolution by natural selection in action.

Peppered Moths are usually white with black speckles covering the wings. These markings make them well camouflaged against lichen covered trees and walls where they rest during the day. A genetic mutation in some moths results in them being melanic (i.e. they have black wings). The black form is not well camouflaged against a lichen covered tree, wall or fence and are, therefore, far more likely to be eaten by predators, such as birds. Under normal environmental conditions fewer of the black type survive to breed and they are, thus, far less common.

During the nineteenth century, following industrialisation, it was noticed in towns and cities that the black form of the moth was becoming more common than the peppered form. The Industrial Revolution and increasing number of coal fires led to the atmosphere becoming increasingly polluted killing off lichen and blackening the tree trunks. The melanic form of the moth was now better camouflaged from predators than the peppered form and thus more likely to survive and breed. As moths are short-lived evolution by natural selection happened quickly.

In the mid twentieth century measures were introduced to control and reduce air pollution and the air in cities and towns became cleaner. Lichens re-colonised tree trunks and walls and the melanic form was now at a disadvantage and more likely to be killed and eaten. The situation now reversed and once again the better camouflaged peppered form of the moth became more abundant in cities.

Unfortunately, the species is now declining and the numbers of Peppered Moths had declined by two thirds between 1968 and 2002 - the reasons for the decline are unknown.