The peppered moth, Biston betularia, is a species whose dramatic color change offers one of the most observable examples of natural selection and rapid adaptation. It is characterized by two primary color forms: a light, speckled variety (typica), and a nearly black variety (carbonaria). The species’ shift between these two morphs in response to human-induced environmental change gave rise to the scientific term “industrial melanism.” Studying this moth provides insight into how populations evolve when faced with intense selective pressure.
Camouflage Before Industrialization
Before widespread industrialization in England, tree trunks were covered in pale lichens. The light-colored typica moth, with its finely speckled wings, was the dominant form. Its coloration provided effective cryptic camouflage against the white and grayish-green bark. This adaptation was crucial because moths spend daylight hours resting motionless on trees. The light coloration rendered them virtually invisible to visual predators, allowing them to survive and reproduce. The dark carbonaria form was exceptionally rare, typically less than 1% of the population, because its dark appearance made it highly conspicuous on pale surfaces.
The Rapid Spread of Melanism
The Industrial Revolution in the mid-19th century drastically altered the moth’s habitat, particularly in urban areas. Soot and smoke from coal-burning factories killed the lichens and coated tree trunks in black grime. This environmental darkening instantly made the light typica form highly visible. The dark carbonaria form, which had been a liability, suddenly gained a selective advantage by blending seamlessly with the sooty bark. The first sighting of this melanic form was in Manchester in 1848, and its spread was swift. By 1895, the dark form reached approximately 98% of the Manchester population. This rapid shift was driven by a single, dominant genetic mutation, identified as a transposable element insertion within the cortex gene.
Predation as the Driving Force
The mechanism driving this shift was differential survival, with avian predators acting as the primary selective agent. Birds, such as robins and thrushes, hunt by sight and preferentially prey upon moths that stand out against the background. In polluted areas, exposed light moths were consumed at a higher rate than camouflaged dark moths. Classic mark-release-recapture experiments conducted by Bernard Kettlewell in the 1950s provided evidence of this selective predation. His work showed that in a polluted wood near Birmingham, light moths were eaten more frequently. Conversely, in an unpolluted area of Dorset, dark moths were selectively removed. These experiments demonstrated that the moth’s coloration dictated its survival probability, confirming that crypsis, or camouflage, was the effective adaptation.
The Modern Reversal of Selection
Adaptation in the peppered moth is not static; the species has demonstrated an ability to evolve in response to subsequent environmental changes. Beginning in the mid-20th century, countries like the United Kingdom implemented clean air legislation, such as the Clean Air Acts. This legislation reduced the soot and sulfur dioxide pollution that characterized the industrial era. As air quality improved, the black grime disappeared, and pale lichens started to recolonize the bark. This reversal of habitat color also reversed the selective pressure. The dark carbonaria form became the conspicuous morph, making it vulnerable to visually hunting birds. Consequently, the frequency of the light typica form has steadily increased in former industrial areas, illustrating that the adaptation’s advantage depends entirely on prevailing environmental conditions.