Life on Earth involves continuous change, with organisms developing features to thrive in their surroundings. This interaction leads to various forms of adaptation. Over time, these adaptations can accumulate, resulting in significant shifts within populations and the emergence of new characteristics. This adjustment is a fundamental aspect of how biological diversity arises and persists.
Understanding Industrial Melanism
Industrial melanism describes when darker-colored animals, particularly moths and other arthropods, become more common in regions affected by industrial pollution. This effect is driven by environmental changes, such as soot and sulfur dioxide, which darken surfaces like tree bark and kill light-colored lichens. Darker pigmentation provides a survival advantage, enabling better camouflage from predators. This shift represents an adaptation unfolding over generations in response to human-induced environmental modifications.
The Peppered Moth Story
The peppered moth, Biston betularia, is a well-known example of industrial melanism. Before Britain’s Industrial Revolution, most of these moths were light-colored with dark speckles (Biston betularia f. typica), providing effective camouflage against lichen-covered tree trunks. A genetic mutation resulted in a dark, almost black, form (Biston betularia f. carbonaria), which was rare because it stood out against light bark, making it vulnerable to predators.
As the Industrial Revolution progressed, coal-burning factories released soot and smoke, blackening tree trunks and killing lichens across industrial regions, such as London and Manchester. This environmental change altered the moths’ habitat; light moths became conspicuous against the darkened surfaces. Conversely, the dark melanic moths, once at a disadvantage, blended with the soot-covered bark. The first dark peppered moth was sighted in Manchester in 1848; by 1895, the dark form comprised 98% of the peppered moth population in that city.
Natural Selection at Play
The change in the peppered moth population illustrates natural selection. The soot-darkened trees created a strong selective pressure. Predatory birds, hunting by sight, found it easier to spot and consume the light-colored moths resting on dark backgrounds. Dark-colored moths were less visible to these predators, significantly increasing their chances of survival.
This differential survival meant dark moths were more likely to reproduce and pass on their genes for dark coloration. Over successive generations, the dark phenotype gene increased rapidly within the moth populations in industrialized areas. This process, where advantageous traits become common in a population due to environmental pressures, directly demonstrates natural selection driving evolutionary change.
Beyond Pollution: Reversal and Significance
Following clean air legislation in the mid-20th century, air quality in industrial regions began to improve. As pollution levels decreased, tree trunks lightened, and lichens recolonized the bark. This environmental shift led to “reverse industrial melanism.” The lighter peppered moth once again gained a selective advantage, becoming better camouflaged against cleaner, lighter tree surfaces.
Consequently, the dark melanic form declined, and light-colored moths became more common again in these areas. For example, in Caldy Common near Liverpool, the dark form decreased from 94% in 1960 to 19% by 1994. The peppered moth story is a compelling, observable example of evolution by natural selection, providing clear evidence for how populations can adapt to changing environmental conditions.