Industrial melanism describes a striking evolutionary process where animal populations, particularly certain moth species, develop darker coloration in environments altered by industrial pollution. This phenomenon serves as a visible example of how organisms undergo rapid adaptive changes in response to shifts in their surroundings. It highlights the direct influence of environmental pressures on natural selection, demonstrating evolution within a relatively short timeframe. This darkening, known as melanism, became prevalent in regions where industrial activities released significant amounts of soot into the atmosphere.
The Classic Example: Peppered Moths
The peppered moth, Biston betularia, provides the most iconic illustration of industrial melanism. Before the 19th-century Industrial Revolution in England, the vast majority of peppered moths were light-colored, featuring a speckled pattern that provided camouflage against pale, lichen-covered tree trunks where they rested. A rare, naturally occurring dark or melanic form, known as carbonaria, existed but was seldom seen due to its visibility. As industrialization progressed, coal-burning factories released immense quantities of soot, which coated tree bark and killed off light-colored lichens across vast landscapes.
This environmental change dramatically altered the moths’ habitat. The light-colored moths, once camouflaged, became highly conspicuous against the now-darkened tree trunks, making them easy targets for bird predators. Conversely, the dark carbonaria moths, previously disadvantaged, now blended seamlessly with the sooty surfaces. This shift in camouflage led to a rapid increase in the proportion of dark moths, with observations in Manchester showing the carbonaria form rising from a rarity to approximately 98% of the population by 1895. Studies consistently demonstrated that bird predation was the primary selective force driving these population changes.
The Driving Forces of Change
The rapid shift in moth coloration was driven by environmental pollution and natural selection. Industrial emissions, primarily soot and sulfur dioxide, drastically transformed ecosystems. These environmental alterations created a strong selective pressure on the moth populations.
In this altered environment, darker moths were more likely to evade predators, reproduce, and pass on their genetic characteristics. The genetic basis for the melanic form in peppered moths is often attributed to a dominant allele. Recent research identified a specific genetic change within the cortex gene, involving a “jumping gene” mutation, as responsible for the dark pigmentation. This genetic advantage allowed the frequency of the dark allele to increase quickly within moth populations, illustrating how environmental changes can lead to swift evolutionary responses.
Reversal and Broader Implications
The story of industrial melanism did not end with the rise of dark moths; it continued with a remarkable reversal. With the implementation of clean air legislation in many industrialized nations from the mid-20th century onward, industrial pollution significantly decreased. As air quality improved, tree trunks gradually lightened as soot deposits diminished and lichens began to recolonize. This environmental recovery shifted the selective pressure once again.
The lighter forms of the moths regained their camouflage advantage, while the dark forms became more visible against cleaner backgrounds. Consequently, a “reverse” industrial melanism occurred, with the frequency of light-colored moths increasing in formerly polluted areas. For instance, in Michigan and Pennsylvania, the prevalence of melanic moths dropped from 90% in 1959 to just 6% by 2001. Similarly, around Liverpool, the dark form declined from over 90% in 1960 to less than 10% by 1994. This dynamic change underscores industrial melanism as a powerful, directly observable instance of evolution in action. It demonstrates how species adapt to fluctuating environmental conditions, making it a significant case study in evolutionary biology.