The peppered moth, Biston betularia, offers an illustration of evolutionary change, particularly between 1848 and 1900. This species has become a classic case study, demonstrating how populations adapt to environmental shifts. Its observable coloration changes provide insights into the forces shaping species over generations.
The Original Peppered Moth
Before the mid-19th century, the typical peppered moth, Biston betularia var. typica, was predominantly light-colored with scattered dark speckles, giving it a “salt-and-pepper” appearance. This coloration provided effective camouflage against light-colored, lichen-covered tree trunks and branches, making them difficult for predators to spot.
This light form was abundant in natural environments, well-hidden from visual predators such as birds. The rare dark variant, or melanic form, existed in very low frequencies, as its dark appearance made it conspicuous against the pale bark and lichens.
The Industrial Revolution’s Environmental Shift
The 19th century brought the Industrial Revolution, fundamentally altering the natural landscape of Britain. The widespread burning of coal to power factories and heat homes released vast quantities of soot and smoke into the atmosphere. This air pollution blanketed the countryside, especially in industrial centers like Manchester and London.
The thick layers of soot darkened the surfaces of trees and buildings. The sulfur dioxide emitted from coal combustion proved toxic to lichens, causing them to die off in polluted areas. This environmental transformation stripped trees of their pale, mottled coverings, leaving behind stark, blackened bark.
The Dramatic Population Change
Between 1848 and 1900, a shift occurred in peppered moth populations within industrial regions. The dark-colored variant, Biston betularia var. carbonaria, once rare, increased significantly in frequency. The first recorded sighting of a dark peppered moth in Manchester was in 1848.
By the end of the 19th century, around 1895, the melanic form dominated moth populations in heavily industrialized areas, reaching frequencies as high as 98% in some locations. This increase in dark moths was accompanied by a decline in the light-colored typica form, which became uncommon in these polluted environments.
A Classic Example of Natural Selection
The observed population shift in the peppered moth is a classic instance of natural selection in action. In soot-darkened environments, light-colored moths, once camouflaged, became highly visible against blackened tree trunks, making them easy targets for predatory birds. Conversely, dark-colored moths, previously conspicuous, now blended effectively with the sooty bark.
As a result, dark moths had a survival advantage, allowing them to live longer and reproduce more successfully. Over generations, genes for dark coloration became more prevalent, a phenomenon termed “industrial melanism.” This demonstrates how environmental pressures drive differential survival and reproduction, leading to rapid evolutionary change within a species.