The peppered moth, Biston betularia, is a common insect found across the Northern Hemisphere, including parts of Asia, Europe, and North America. This night-flying moth is recognized for its varying body coloration.
The Moth’s Natural Camouflage
The original and widespread form of the peppered moth, known as the typica morph, exhibits a light, speckled appearance. Its wings are whitish or ash-grey, adorned with numerous small black spots, resembling scattered pepper. This coloration provides excellent camouflage against tree bark covered with light-colored lichens, prevalent in unpolluted environments. When resting on such surfaces, the typica moth blends seamlessly into its surroundings, making it difficult for predators, primarily birds, to spot. This concealment offers a significant survival advantage, helping them avoid predation and reproduce.
The Emergence of Darker Moths
During the 19th-century Industrial Revolution in England, a significant shift occurred in peppered moth populations. Factories burning coal released vast amounts of soot and sulfur dioxide, blackening tree trunks and killing off light-colored lichens.
In this altered environment, a previously rare dark-colored morph, known as carbonaria, began to thrive. This melanic form is almost entirely black or dark grey, providing superior camouflage against the soot-darkened trees.
The light-colored typica moths, once camouflaged, became conspicuous against the dark bark, making them easy targets for birds. Conversely, the dark carbonaria moths were now well hidden, allowing more of them to survive and reproduce. This selective advantage led to a rapid increase in the dark morph’s frequency; by 1895, the carbonaria form reached 98% of the peppered moth population in heavily industrialized areas like Manchester. This phenomenon, where dark pigmentation evolves in response to industrial pollution, is known as industrial melanism.
Recent Shifts in Moth Populations
Following the Industrial Revolution, many industrialized nations implemented clean air legislation, such as the Clean Air Act in the United Kingdom in 1956. These regulations significantly reduced air pollution, leading to cleaner environments. As soot levels decreased, lichens began to recolonize tree trunks, and the bark lightened.
This environmental recovery reversed the selective pressure on the peppered moth populations. The dark carbonaria form, once camouflaged, now stood out against the cleaner, lighter tree surfaces, while the light typica form regained its camouflage advantage. Consequently, the frequency of the light-colored moths has increased in many areas, demonstrating natural selection and the moth’s adaptation.
A Living Example of Evolution
The color changes observed in peppered moth populations serve as an illustration of evolution driven by natural selection. The moth’s ability to adapt rapidly to environmental changes, first by becoming darker during pollution and then lightening as conditions improved, highlights how traits within a population can shift over time. This species continues to be a subject of study in evolutionary biology, showcasing how selective pressures can influence the survival and prevalence of different forms within a species. The variations in moth coloration represent a direct response to the pressures of their surroundings.