The peppered moth (Biston betularia) serves as a compelling and widely recognized example of evolutionary change observed in real-time. This species, found across Europe, Asia, and North America, has captivated scientists and the public due to the dramatic shifts in its coloration over the past two centuries. The story of the peppered moth illustrates how environmental pressures can drive rapid adaptations within a species. Understanding the historical prevalence of its different forms helps to unravel this biological narrative.
The Peppered Moth Before the Industrial Revolution
Over 150 years ago, prior to the Industrial Revolution, the light-colored form of the peppered moth, known scientifically as typica, was highly prevalent. These moths featured a mottled black and white pattern, giving them a “salt-and-pepper” appearance. This coloration provided effective camouflage against light-colored, lichen-covered tree trunks and branches where they rested during the day. Predators, primarily birds, found it difficult to spot these well-blended moths against their natural background, allowing the light form to thrive.
In contrast, the dark-colored variant, or carbonaria form, was exceedingly rare. While a genetic mutation produced these darker individuals, their almost entirely black bodies made them highly conspicuous against the pale, lichen-covered trees. This lack of camouflage meant that dark moths were much more vulnerable to bird predation, significantly reducing their survival rates. The first recorded dark specimen was collected before 1811, indicating its presence but extreme rarity.
The Impact of Industrialization
The Industrial Revolution in the late 18th and 19th centuries dramatically altered the natural environment, particularly in industrialized regions of Britain. Factories burning coal released vast amounts of soot and sulfur dioxide into the atmosphere. This pollution caused the death of light-colored lichens that adorned tree trunks and also blackening the bark with soot deposits.
This environmental transformation reversed the camouflage advantage the light-colored moths once held. Against the now-darkened, bare tree trunks, the typica form became highly visible to predatory birds. Conversely, the once-rare dark carbonaria moths blended seamlessly with the soot-stained bark. This shift in camouflage led to a rapid increase in the dark moth population, as they were better able to evade predators and survive to reproduce. By 1895, the dark form accounted for 98% of peppered moths in heavily industrialized areas like Manchester.
Natural Selection: The Driving Force
The observed shift in peppered moth coloration is a classic demonstration of natural selection, a fundamental mechanism of evolution. Natural selection describes how environmental pressures favor individuals with traits that enhance their survival and reproduction. For the peppered moth, predation by birds served as the primary selective pressure.
Before industrialization, light-colored moths possessed a survival advantage due to their camouflage, making them more likely to pass on their genes. When pollution darkened the trees, the advantage shifted to dark-colored moths. Birds more easily spotted and consumed light moths, while dark moths survived and reproduced at higher rates. This differential survival, driven by the changing environment, led to a rapid increase in the frequency of the gene responsible for dark coloration within the moth population.
The Peppered Moth Today
In the latter half of the 20th century, stricter pollution control measures, such as Britain’s Clean Air Act of 1956, significantly improved air quality. As the air became cleaner, soot levels decreased, and lichens began to recolonize tree trunks. This environmental reversal once again altered the selective pressures on the peppered moth.
With the return of lighter, lichen-covered trees, the camouflage advantage reverted to the light-colored typica form. Consequently, the population of light moths has increased, while the dark carbonaria form has declined in many regions. This recent shift further reinforces the peppered moth’s role as a compelling, ongoing example of natural selection.