The peppered moth, Biston betularia, presents a remarkable biological phenomenon observed over the past two centuries. This species exists primarily in two forms: a light, speckled variety and a dark, almost black variety. Scientists have documented a dramatic shift in the prevalence of these two forms within moth populations. This change in coloration highlights how living organisms can adapt to their surroundings.
The Moths Before Industrialization and Environmental Shift
Before the Industrial Revolution, the light-colored peppered moth was the predominant form across Britain. Its pale, speckled wings provided effective camouflage against tree trunks covered with light-colored lichens and bark. Moths rest on tree trunks during the day, making their coloration a crucial factor for concealment from predators. The light moths blended seamlessly into their environment, making them difficult for birds to spot.
The late 18th to mid-19th centuries marked the onset of the Industrial Revolution, particularly in the United Kingdom, leading to widespread environmental changes. Factories powered by coal burning released vast amounts of smoke and soot into the atmosphere. This air pollution resulted in a thick layer of black soot covering tree trunks and other surfaces in industrial areas. The sulfur dioxide emissions also killed off the sensitive, light-colored lichens, leaving the bark exposed and dark. This alteration transformed the moths’ resting environment from light to dark.
Natural Selection in Action
The environmental shift created a significant selective pressure on the peppered moth population, leading to industrial melanism. As tree trunks darkened with soot and lost their lichens, the light-colored moths, once camouflaged, became conspicuous. Conversely, the rare dark-colored moths, previously more visible, now blended in effectively with the blackened trees. This reversal of camouflage advantage was evident in industrial regions, such as Manchester, England.
Predators, primarily birds, played a direct role in driving this change. Birds more easily spotted and consumed the light-colored moths resting on the darkened trees. This differential predation meant that dark moths had a higher survival rate, allowing them to live longer and reproduce more successfully. Consequently, they passed on their dark-colored genes to more offspring, leading to a rapid increase in the dark moth population. The genetic basis for this dark coloration is controlled by a single gene.
By the late 19th century, the dark form, carbonaria, had become prevalent in polluted areas, reaching frequencies as high as 98% in places like Manchester by 1895. This shift demonstrated natural selection, where the environment favored specific traits, resulting in rapid evolutionary change within the species.
The Reversal of Color Change
In the mid-20th century, particularly in the United Kingdom, efforts were made to combat air pollution. Clean air legislation aimed to reduce industrial emissions and improve air quality. These acts mandated cleaner burning fuels and imposed controls on smoke and grit from factories and domestic sources.
As a direct result of these clean air initiatives, pollution levels decreased, and the environment began to recover. Tree trunks gradually became cleaner, and the light-colored lichens started to return to the bark. This environmental cleanup reversed the selective pressure that had favored the dark moths.
With the trees lightening, the light-colored peppered moths once again gained a survival advantage, blending in better with their surroundings. The dark moths, now more visible against the cleaner bark, became more susceptible to bird predation. This led to a decline in the dark moth population and a resurgence of the light form. The peppered moth’s changing coloration remains an example of how populations adapt to environmental shifts through natural selection.