The Peppered Moth (Biston betularia) is a species whose color variation became a classic example of rapid evolutionary change. It has two primary color forms, light and dark, which historically experienced a dramatic shift in prevalence. This shift showcases how populations can quickly adapt when the environment imposes a strong selective pressure on inherited traits.
The Pre-Industrial Moth and Its Camouflage
Before the environmental changes of the 19th century, the vast majority of the moth population was the light-colored, speckled form, typica. This coloration, featuring a white background dusted with small black spots, provided near-perfect concealment. Moths rested on tree trunks and branches typically covered in pale, crusty lichens.
The light, mottled pattern allowed the typica form to blend seamlessly with the lichens and unpolluted bark. This effective camouflage was a survival mechanism against visual predators, primarily insectivorous birds. Because the light moths were difficult for birds to see, they had a higher chance of surviving to reproduce and pass their genes to the next generation.
The Environmental Catalyst of the Industrial Revolution
The environmental balance favoring the light moth was disrupted by the Industrial Revolution in Britain. Widespread burning of coal in factories released soot and sulfur dioxide into the atmosphere. This air pollution served as the trigger for the moth’s color change.
Sulfur dioxide emissions were toxic to the delicate lichens covering the tree bark, causing them to die off. Simultaneously, heavy coal soot coated the tree trunks and branches in a thick layer of black carbon. This fundamentally altered the moth’s resting background from a pale, speckled canvas to a uniformly dark one, exposing the formerly camouflaged light moths.
Natural Selection and Differential Survival
The change in environment created a powerful new selective pressure favoring the rare dark-colored moth, the carbonaria form. This dark coloration is due to melanism, a genetic trait that existed at very low frequencies before industrialization. Against the soot-blackened trees, the light typica moths stood out starkly, making them easy targets for predatory birds.
The dark carbonaria form suddenly gained a significant survival advantage, as its color provided excellent camouflage against the dark bark. This difference in visibility led to differential survival: dark moths were less likely to be eaten and more likely to survive and reproduce.
The melanic trait is controlled by a dominant allele, allowing it to spread rapidly once the environment favored it. Within a few decades, the frequency of the dark form in polluted areas, such as Manchester, rose dramatically. By 1895, the dark form constituted nearly 98% of the moth population, demonstrating rapid adaptation via natural selection.
The Reversal of Coloration
The dominance of the dark moth form did not last forever, as subsequent human actions led to another environmental shift. Beginning in the mid-20th century, legislation like the Clean Air Acts was implemented across Britain to reduce smoke emissions. The resulting reduction in air pollution led to the gradual cleaning of the environment.
As pollution levels dropped, soot washed away and sensitive lichens began to recolonize the tree trunks. This process reversed the background color, making the trees lighter and more speckled again. The light typica form was once again better camouflaged, while the dark carbonaria form became more conspicuous to predators. This reversal of selective pressure caused the frequency of the dark moth to decline steadily in cleaner areas. The resurgence of the light form confirmed the direct link between the moth’s color and its habitat’s environmental conditions.