The peppered moth (Biston betularia) is a night-flying species recognized as a classic example of evolution. It is distinguished by variable, speckled wing patterns that normally provide effective camouflage against tree bark. Its importance stems from the rapid, observable changes in coloration frequencies during and after the Industrial Revolution, demonstrating how natural selection acts powerfully in response to environmental shifts.
Geographic Range and Preferred Habitat
The peppered moth has a broad geographic distribution across the temperate regions of the Northern Hemisphere (Holarctic distribution). This range includes North America, Europe, and Asia, stretching coast-to-coast in many areas. The moth’s presence is tied directly to environments that support its larval host plants.
It is most commonly found in deciduous woodlands and mixed forests, but it also thrives in woodlots, hedgerows, and urban parks and gardens. The larvae are generalist feeders, consuming the foliage of numerous host trees and shrubs, including birch, willow, oak, and elm. Adult moths spend the day resting on tree trunks or branches.
How Local Environment Influences Moth Appearance
The moth is characterized by two forms: the typical, light-colored morph (f. typica) and the dark, melanic morph (f. carbonaria). Before the Industrial Revolution, the light form predominated, its speckled wings blending perfectly with the pale, lichen-covered bark of trees. The melanic form was extremely rare, often making up less than 0.01% of the population in unpolluted areas.
The selective pressure driving the change is predation, primarily by birds, which hunt by sight. When industrialization blanketed cities and forests in soot and sulfur dioxide, the pollution killed the lichens and blackened the tree bark. The light-colored moths resting on the bare, dark bark became highly visible targets for predators.
The previously rare dark form was now better camouflaged against the sooty background. This shift in environmental conditions gave the melanic moths a survival advantage. This phenomenon, known as industrial melanism, caused a dramatic increase in the dark form’s frequency, reaching as high as 98% in heavily polluted areas like Manchester, England, by 1895.
Current Population Frequencies Following Environmental Changes
The evolutionary success of the dark moth was reversed in the mid-20th century following the implementation of clean air legislation. These acts drastically reduced the amount of soot and sulfur dioxide released into the atmosphere. The improved air quality allowed lichens to regrow on tree trunks, restoring light-colored resting surfaces.
The selective advantage swung back toward the light-colored moth. The dark melanic form became conspicuous against the pale, lichen-covered bark, resulting in increased predation by birds. Studies tracking the moth’s population have confirmed this shift, a process often termed “reverse industrial melanism.”
In the Manchester area, for instance, the frequency of the dark morph has fallen significantly, dropping from approximately 90% in the 1980s to below 10% in recent decades. The light-colored morph is now the dominant form in formerly polluted regions. The population frequencies continue to reflect the local air quality, providing a direct, observable link between environmental conditions and evolutionary change.