Industrial Melanism in Pepper Moths (Biston betularia)

Prior to the Industrial Revolution of the mid-1800s in Great Britain, birch and other tree species were typically covered with lichens, which produced a mottled pattern against which white Pepper Moths (Biston betularia) were well-camouflaged [upper left]. Insect collections from the period show a small number of melanic (black) forms [upper right], which were thought to be a distinct species, carbonaria. The difference turns out to be allelic variation at a single locus, with the black alleles dominant to white. As the Industrial Revolution progressed, factories in the English Midlands (Birmingham and elsewhere) produced large quantities of soot that settled on trees and blackened large sections of forest. Against such environmental backgrounds, the melanic form is at a selective advantage over the white form with respect to avian visual predators [bottom]. [This can be shown experimentally]. The frequency of the melanic form increased rapidly (as shown by insect collections) and nearly replaced the white form in some areas. Pollution control measures, including scrubbers on smokestacks, beginning in the mid-20th century, allowed replacement of the blackened trees with new forests of white trees. The white morph once again gained a selective advantage over the melanic form, and has increased in numbers.

Natural Selection on the Pepper Moth is one of the outstanding example of Ecological Genetics, and has been studied for many decades.