The peppered moth, scientifically known as Biston betularia, is a temperate species of nocturnal moth native to Europe, Asia, and North America. This insect, which spends its days resting on tree trunks and branches, has become widely recognized for the dramatic, observable changes in its coloration. The peppered moth provides one of the most compelling and frequently cited real-world examples of Charles Darwin’s theory of natural selection. Its story illustrates how environmental shifts can quickly alter the frequency of inherited traits, leading to measurable evolutionary change.
Physical Characteristics and Classification
The peppered moth belongs to the family Geometridae (geometer moths) and is a member of the insect order Lepidoptera. As an adult, the moth has a stout body and a wingspan that typically ranges from 45 to 62 millimeters. Its name comes from the traditional appearance of the light-colored form, known as typica, which features pale ash-gray wings speckled with numerous fine black markings. This mottled pattern provides superb camouflage against the light-colored, lichen-covered bark of trees. The species is polymorphic, meaning it naturally occurs in different color forms. The dark form, called carbonaria, is almost entirely sooty black.
The Evolutionary Story of Industrial Melanism
The moth’s notoriety stems from industrial melanism, which occurred following the widespread pollution of the Industrial Revolution, primarily in 19th-century Britain. Before this period, the light typica form dominated populations because its markings blended perfectly with the lichen-covered trees. A rare, dark carbonaria mutant existed but was easily spotted by avian predators against the pale background, keeping its frequency low.
Coal-burning factories released massive amounts of soot, which coated tree trunks and killed the pale lichens, effectively turning the moth’s environment dark. This drastic change reversed the survival advantage: the light typica moths were now conspicuous targets for birds, while the dark carbonaria moths became camouflaged against the blackened bark. The selective pressure of predation caused the frequency of the dark form to rise dramatically, reaching as high as 98% in heavily industrialized areas like Manchester by 1895.
The genetic basis for this rapid adaptation is controlled primarily by a single dominant gene. Molecular studies pinpointed the cause of the melanic trait to the insertion of a transposable element, or “jumping gene,” into the first intron of the cortex gene. This genetic change, which occurred around 1819, increased the production of a transcript involved in wing development, leading to the dark pigmentation. The swift rise of the carbonaria form demonstrated a measurable instance of natural selection favoring a pre-existing beneficial mutation.
Life Cycle and Natural History
The peppered moth undergoes complete metamorphosis, passing through four distinct life stages: egg, larva, pupa, and adult. Adult moths are generally active between May and August, emerging from their pupal cases in the spring or early summer. They are strictly nocturnal, spending the daylight hours motionless on trees to avoid detection by diurnal predators.
After mating, the female moth lays hundreds of tiny, pale-green eggs into crevices in bark or under lichens. The larvae, or caterpillars, hatch in mid-summer and feed on the leaves of a variety of deciduous trees, including oak, birch, and willow. The caterpillar exhibits remarkable camouflage, mimicking the appearance of a small, brown or green twig, a defense mechanism known as twig mimicry. As autumn approaches, the fully grown larva descends to the ground and burrows into the soil or leaf litter to pupate, spending the entire winter dormant until the weather warms.
Current Population Trends
The environmental conditions that drove industrial melanism began to reverse in the mid-20th century, following the implementation of Clean Air Acts in countries like the United Kingdom and the United States. These legislative measures significantly reduced atmospheric soot and sulfur dioxide pollution, allowing lichens to recolonize tree trunks and bark to lighten in color. This ecological recovery immediately reversed the selective advantage.
With lighter backgrounds becoming common again, the dark carbonaria form was once more vulnerable to detection and predation by birds. Consequently, the light typica form regained its camouflage advantage. Monitoring in formerly polluted regions confirmed a rapid decline in the frequency of the dark morph. For instance, in the Manchester area, the carbonaria frequency plummeted from its peak near 98% to less than 10% in some populations, demonstrating a measurable evolutionary reversal. The ongoing decline of the dark form illustrates how natural selection is a dynamic process, constantly tracking and responding to shifts in the surrounding ecosystem.