The story of the dark moths, a textbook example of evolution in action, is centered on the Peppered Moth, Biston betularia. This species underwent a visible color change during the Industrial Revolution, providing a clear demonstration of how a shift in environment can drive rapid biological change. The first recorded instance of the dark form was collected near Manchester, England, in 1848, setting the stage for a biological phenomenon driven by coal smoke and industrial pollution.
The Rise of Industrial Melanism in Moths
The biological phenomenon observed in the Peppered Moth is known as industrial melanism, describing the evolutionary development of dark pigmentation in populations inhabiting polluted areas. Before the mid-19th century, the vast majority of the species was the light-colored, speckled form, known as typica. This pale coloration allowed the moths to blend seamlessly with the light-colored, lichen-covered tree bark where they rested.
A naturally occurring genetic mutation produced a dark, almost solid black variant known as the carbonaria form. This dark form was rare in the unpolluted pre-industrial environment because its lack of camouflage made it conspicuous to predators. As industrialization intensified, the carbonaria form quickly became the dominant morph in polluted regions, demonstrating a swift evolutionary response. This shift defines industrial melanism, which was documented in over a hundred different moth species during this period.
Peak Geographical Range of Dark Moths
The dark moths were concentrated heavily in the industrialized regions of England, corresponding precisely to areas where coal-burning factories blanketed the landscape in soot and smoke. The first specimen was found in Manchester, which became an epicenter of the phenomenon.
By the late 19th century, the melanic form reached frequencies as high as 98% near Manchester. Other major industrial centers, including Liverpool and Birmingham, also showed high populations of the dark form. The correlation between the density of the carbonaria form and localized atmospheric pollution, particularly sulfur dioxide, was unmistakable.
While the dark form spread throughout the North and East of England, rural areas remained largely unaffected. Regions like Dorset and parts of Scotland, which lacked heavy industrial pollution, retained populations dominated by the original light typica form. The highest frequency of dark moths was a clear geographical marker of Britain’s industrial heartland.
The Selective Pressure Driving Color Change
The mechanism behind the color change was natural selection, driven by bird predation. Soot from coal combustion killed the pale lichens on tree trunks and darkened the underlying bark. This environmental change reversed the camouflage advantage for the two moth morphs.
The previously hidden light typica moths suddenly became highly visible against the black, bare tree trunks. Conversely, the dark carbonaria form was now perfectly camouflaged against the soot-covered surfaces. Avian predators preferentially removed the conspicuous light forms in polluted areas.
Entomologist Henry Bernard Davis Kettlewell confirmed this selective pressure with classic experiments in the 1950s. He performed mark-release-recapture studies in both polluted woodlands near Birmingham and unpolluted areas in Dorset. His results showed that camouflage determined survival, as more dark moths were recaptured in polluted areas and more light moths survived in clean environments.
The Reversal of Moth Coloration in Modern Times
The historical narrative of the dark moths continued past the peak of industrial pollution in the early 20th century. Following World War II, a significant environmental reversal began with the introduction of clean air legislation. The British Parliament passed the Clean Air Act of 1956, aiming to reduce atmospheric pollution, especially in urban centers.
As coal smoke and sulfur dioxide levels decreased, the environment began to lighten. Lichens returned to the tree trunks, and bark surfaces became cleaner, reversing the dark background that had favored the carbonaria form. This environmental shift immediately reversed the selective advantage.
The light typica form once again became the better-camouflaged morph, leading to a rapid decline in the frequency of the dark moths across Britain. In many industrial areas, the carbonaria form’s frequency plummeted to levels comparable to those seen before the Industrial Revolution. This decline provided a second, powerful demonstration of natural selection responding to environmental change.