Industrial melanism is the increase in dark-pigmented, or melanic, individuals within a population of organisms, most notably moths and other insects. This phenomenon is a clear example of rapid evolution in direct response to human-caused environmental changes. The darkening occurs in areas affected by heavy industrial pollution, illustrating how organisms adapt to survive in altered habitats globally.
The Evolutionary Mechanism of Color Change
The core driver of industrial melanism is the change in the background environment, which alters the camouflage effectiveness of different color morphs. Before the Industrial Revolution, trees in Britain and other areas were covered in light-colored lichens, making the pale, speckled form of various moth species virtually invisible to predators. The widespread burning of coal released massive amounts of soot and sulfur dioxide. Sulfur dioxide killed the pale lichens, leaving the tree bark exposed and dark.
The resulting soot deposited on the bark and other surfaces, creating a consistently dark backdrop. This environmental shift reversed the selective pressure: the previously rare dark-pigmented moths, which possessed a genetic mutation for increased melanin production, were now better camouflaged. Avian predators more easily spotted and consumed the lighter, typical morphs resting on the blackened surfaces. This differential predation allowed the darker morphs to survive and reproduce at a higher rate, leading to a swift increase in their frequency within the population.
The Landmark Case: British Peppered Moths
The most thoroughly documented instance of industrial melanism centers on the Peppered Moth, Biston betularia, in Great Britain. The original pale, speckled form, known as typica, dominated populations before the mid-19th century. The first recorded dark morph, called carbonaria, was captured in Manchester in 1848, coinciding with the peak of industrial activity.
Over the next five decades, the frequency of the dark carbonaria form exploded in the industrial heartlands. In major industrial centers like Manchester and Birmingham, the melanic form rose from being rare to making up more than 90% of the moth population by the late 1800s. Geneticist Bernard Kettlewell later conducted field experiments in the 1950s using mark-release-recapture techniques. His work provided direct evidence that selective bird predation was the mechanism driving the observed changes in morph frequency.
Documented Instances Across North America and Europe
The evolutionary response of industrial melanism was not confined to Britain or the Peppered Moth. Similar changes were observed in numerous other species of Lepidoptera across industrialized regions of Europe and North America. Over 70 species of moths in Britain alone developed melanic forms during the industrial period.
In continental Europe, melanism in Biston betularia was documented in industrial zones of the Netherlands and Germany. In North America, the native subspecies, Biston betularia cognataria, underwent an almost identical evolutionary trajectory in the eastern part of the continent. Melanic forms became prevalent in industrial cities such as New York City, Philadelphia, Detroit, and Pittsburgh, Pennsylvania.
The phenomenon extended to other moth species, such as the Pale Brindled Beauty moth (Phigalia pilosaria) and the Scalloped Hazel moth (Odontopera bidentata) in Europe. The parallel development of the dark color morph in these different species and continents suggests a shared genetic mechanism favored by the same environmental pressure.
The Reversal of Melanism Following Clean Air Initiatives
The narrative of industrial melanism provided a unique opportunity to observe evolution in reverse. Beginning in the mid-20th century, Western nations enacted clean air legislation, such as the United Kingdom’s Clean Air Acts of 1956 and 1968, to reduce atmospheric soot and sulfur dioxide. This legislation mandated cleaner fuels and controlled industrial emissions, leading to a gradual environmental cleanup.
As pollution levels dropped, lichens began to recolonize the tree trunks, and the dark soot faded from surfaces. This reversal of the background environment shifted the selective advantage back to the lighter, typical morphs. The melanic moths, once camouflaged, were now more conspicuous to predators on the cleaner, lichen-covered bark.
In the Manchester area, the frequency of the dark carbonaria form, which had peaked at over 90%, steadily declined to below 10% in recent decades. Similar declines in melanism have been observed across North America and Europe following improvements in air quality. This measurable decline provides ongoing evidence that the moth populations are continually adapting to the changing environment.