Peppered Moth

The peppered moth (Biston betularia) is a moth that was drastically affected by soot pollution in England in the 1800's and is a classic demonstration of natural selection.

In England in the 1880's, soot covered much of the landscape, following the advent of the Industrial Revolution, which began about 1760 in England. It was characterized chiefly by the replacement of hand tools with power-driven machines, as the power loom and the steam engine, and by the concentration of industry in large establishments.

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Previous to the soot problem, the light-colored variety of the moth was common in England. Its speckled coloration helped camouflage the moth when perched on birch tree trunks (this nocturnal moth rests on birch tree trunks during the day). This type of camouflage is called cryptic coloration. After pollution covered many of the trees with a dark layer of soot and killed the light, whitish-gray colored lichens that lived on the tree trunks, a dark form of the moth was observed. Since the light-colored variety was no longer camouflaged when at rest on trees, and they were caught by birds - the dark-colored variety were now better camouflaged and more likely to survive and reproduce). The dark form of the peppered moth was first observed in 1848, and by 1895, 95 percent of the peppered moths were of the darker type. This phenomenon is called "industrial melanism."

An Unpolluted Birch Forest
The light variety of the peppered moth on a birch tree trunk. /
The dark variety of the peppered moth on a birch tree trunk.
A Polluted Birch Forest
The light variety of the peppered moth on a soot-blackened tree trunk. /
The dark variety of the peppered moth on a soot-blackened tree trunk.

The British biologist E.B. Ford wrote about the peppered moth in 1937 and hypothesized that cryptic coloration explained the prevalence of the few, melanic (darker) form. In the early 1950's, the British doctor H. B. D. Kettlewell studied the phenomenon from a genetic standpoint. He performed mark-and-recapture experiments on the peppered moth, releasing both light and dark moths into either a polluted or an unpolluted forest. After a time, he recaptured moths from both areas. In the polluted forests, he caught more dark moths; in non-polluted forests, he captured more light-colored moths. Kettlewell determined that the melanism was a trait that was inherited as a single, dominant allele (one of many altenrate forms of a gene that is located at a particular place on a chromosome). [Ref. Kettlewell, H. B. D. 1955. Heredity 9: 323-342 and Kettlewell, H. B. D. 1956. Heredity 10: 287-301.

Recently, as cleaner-burning fuels have been used in England, the light-colored variety has returned to prominence.