The dramatic color shift observed in certain moth populations, known as industrial melanism, presented a profound mystery to naturalists in the 19th century. This rapid darkening of wings and bodies, changing from a light-colored, speckled pattern to nearly solid black, occurred simultaneously with the rise of industrial cities. The change was swift and widespread, becoming a compelling, real-time example of a species adapting to environmental transformation.
The Peppered Moth Before the Industrial Age
The species at the center of this change, the peppered moth (Biston betularia), existed predominantly in a light-colored form, known as typica, for centuries before the 1800s. These moths possessed white wings finely dusted with black speckles, giving them a “peppered” appearance. This coloration provided effective camouflage against their natural habitat: tree trunks and branches covered with pale-colored lichens.
In an unpolluted environment, light-colored lichens were abundant, creating a mottled, light-toned resting surface for the moths during the day. Bird predators frequently overlooked the typica form because its pattern blended seamlessly with the lichen-covered bark. Rare, naturally occurring dark individuals were easily spotted and eaten, keeping their numbers low in the population. The light color was thus the favored trait for survival and reproduction in the clean, pre-industrial landscape.
The Environmental Shift and Selective Pressure
The dark color in the moth population was caused by the massive increase in air pollution, specifically coal smoke and soot, released during the 19th-century Industrial Revolution in Britain. Factories and domestic coal fires blanketed the landscape with oily, black particulate matter. This pervasive soot had two immediate effects: it killed the light-colored lichens and stained the bare tree bark a uniform, dark color.
This environmental transformation created a powerful selective pressure on the moth population. The light-colored moths were now highly conspicuous against the darkened tree trunks, making them easy targets for bird predators. Conversely, the dark moths, previously at a disadvantage, suddenly blended in with the soot-blackened bark. This reversal meant the dark moths were more likely to survive and pass their genes, while the light moths were selectively removed by predation.
The frequency of the dark form, known as carbonaria, rose dramatically in industrial areas. For example, the first recorded dark moth was collected near Manchester in 1848, and within 50 years, by 1895, the carbonaria form accounted for approximately 98% of the peppered moth population in that city. This rapid shift is a textbook example of directional selection, where the environment directly favored one extreme trait. The dark color was not created by the pollution, but the pollution conferred a survival advantage to the existing dark trait.
The Genetic Mechanism of Color Change
The dark coloration is controlled by a specific, dominant allele, the carbonaria allele, existing at a single location on the moth’s chromosomes. Because it is dominant, a moth only needs one copy of the allele to exhibit the full dark color. Before industrialization, this allele was present in the population, but its low frequency meant that dark moths were rare.
Recent genetic studies have pinpointed the origin of the carbonaria allele to a large, repeated DNA sequence known as a transposable element, or “jumping gene.” This element inserted itself into the cortex gene around 1819, coinciding closely with the onset of widespread coal burning and air pollution. The presence of this transposable element in the cortex gene caused the moth’s wings to develop solid black pigmentation.
The environmental shift did not cause the mutation itself, but acted as a filter, rapidly increasing the prevalence of the existing carbonaria allele. Once the dark moths gained a survival advantage through camouflage, they reproduced more successfully than the light moths. Over many generations, the frequency of the dominant allele spread quickly through the population in polluted regions.
Modern Reversal and Scientific Validation
The story of the peppered moth continued past the peak of industrial melanism in the mid-20th century. Beginning in the 1950s and 1960s, industrialized countries implemented clean air legislation to reduce coal smoke and particulate emissions. This reduction in pollution led to a corresponding lightening of the environment, as soot dissipated and lichens began to recolonize the tree trunks.
As the tree trunks returned to their pale, mottled appearance, the selective advantage of the dark carbonaria form vanished. The dark moths were now easily visible against the clean bark, while the light-colored typica form was once again well-camouflaged. This environmental reversal caused the population to shift back, with the dark morph becoming less common and the light morph experiencing a rapid rebound.
In some areas, the frequency of the dark moth dropped from over 90% to below 20% in just a few decades, correlating strongly with the reduction in air pollution. This “evolution in reverse” provided powerful evidence that the dark color was caused by a genetic trait favored by a specific, transient environmental factor. The peppered moth remains one of the most compelling examples of natural selection responding to human-induced change.