Industrial melanism describes an evolutionary process where dark-colored forms of animal species, particularly insects, become more common in environments affected by industrial pollution. This phenomenon occurs in areas where soot and other airborne particles have darkened surfaces like tree bark, providing a camouflage advantage to darker individuals. It represents a visible example of how environmental changes can drive rapid adaptive shifts within populations. This process is distinct from other forms of animal coloration, specifically linked to human-induced alterations of habitats.
The Classic Example: Peppered Moths
The most widely studied instance of industrial melanism involves the peppered moth, Biston betularia, in Great Britain. Before the mid-19th century, the light-colored, speckled form of the moth, known as typica, was predominant, effectively blending with lichen-covered tree trunks. The Industrial Revolution brought significant coal burning, releasing vast amounts of soot into the atmosphere. This soot settled on trees, turning their bark dark and also killing off much of the pale lichen that previously covered them.
This environmental shift led to an increase in the frequency of a dark-colored variant, carbonaria, first recorded in Manchester in 1848. The dark moths gained a survival advantage because they were better camouflaged against the soot-darkened trees, making them less visible to avian predators.
Researchers like Bernard Kettlewell conducted experiments in the 1950s, releasing both light and dark moths into polluted and unpolluted woodlands. His observations showed that light moths were more frequently eaten by birds in polluted areas, while dark moths suffered higher predation in unpolluted environments. This provided direct evidence that differential predation drove the changes in moth populations.
The Science Behind the Change
The shift in peppered moth coloration illustrates natural selection. Environmental pressures, such as darkened tree trunks from industrial pollution, exerted a selective force on the moth population. Individuals with advantageous traits in the altered environment were more likely to survive and reproduce.
This survival advantage meant dark-colored moths passed on their genetic traits to more offspring. The dark coloration is primarily controlled by a single gene, with the melanic allele being dominant. Over generations, the frequency of this allele increased, leading to a higher proportion of dark moths. This demonstrates how heritable traits that confer a fitness benefit become more prevalent over time, adapting a species to its changing surroundings.
Modern Observations and Reversal
Following clean air legislation in many industrialized nations, a reversal of industrial melanism has been observed. As air quality improved, soot levels decreased, and lichens recolonized tree trunks, restoring their lighter appearance. This environmental change shifted the selective pressure. Light-colored peppered moths regained their camouflage advantage, and their populations began to rebound.
In Britain, for example, the frequency of the dark carbonaria form has declined since the mid-20th century, with the light typica form becoming dominant again. Similar reversals have been noted in other species and regions where pollution has been reduced, though the peppered moth remains the most prominent example. This contemporary shift underscores the dynamic nature of evolution and how species can adapt quickly to fluctuating environmental conditions, even those caused by human activity.