Industrial Melanism

Industrial melanism refers to the evolutionary phenomenon in which dark-pigmented forms of species become more prevalent in environments affected by industrial pollution. This change occurs primarily through natural selection, as darker individuals gain a survival advantage in polluted habitats stained by soot and altered by sulphur dioxide emissions. The concept has become one of the most cited empirical examples of rapid evolutionary change, especially within the field of evolutionary biology.

Environmental Basis and Mechanism

Industrial pollution, notably the release of sulphur dioxide and the deposition of soot, transforms ecosystems by killing lichens and darkening tree bark and other natural surfaces. In clean environments, lichens create a mottled, pale background against which lighter-coloured species are better camouflaged. However, when lichens die and bark becomes blackened, light-coloured individuals become more visible to predators, reducing their survival.
Under these altered environmental conditions, dark-pigmented, or melanic, individuals benefit from improved camouflage. As they are less easily detected by predators, they display higher fitness and reproduce more successfully. Natural selection thus favours melanic variants, increasing their frequency in polluted areas. The subsequent decline of melanism in many regions following pollution control provides powerful evidence that camouflage-driven selection is the principal mechanism.

Historical Observations and Early Interpretations

The first recorded observations of industrial melanism occurred around 1900, when the geneticist William Bateson noted heritable colour polymorphisms in several moth species but did not infer a specific cause. By 1906 Leonard Doncaster had documented a marked rise in melanic forms of moths in northern England during the early nineteenth century.
A theoretical explanation emerged with the work of J. B. S. Haldane. In 1924 he modelled the rapid increase of the melanic form of the peppered moth (Biston betularia), demonstrating mathematically that strong selection pressures must have been at work long before this was widely accepted.
Systematic experiments began in the mid-twentieth century with the work of Bernard Kettlewell, who used capture–mark–recapture methods to compare survival rates between light and melanic forms. His findings showed that melanic moths survived significantly better in polluted environments, providing one of the earliest experimental demonstrations of natural selection in the wild.

The Peppered Moth and Empirical Evidence

Among the many species affected by industrial melanism, the peppered moth became the most iconic example. Before industrialisation, the typical pale form blended against lichen-covered trees. Industrial pollution reduced lichen coverage and darkened the bark, reversing the advantage: melanic individuals achieved up to a 30 per cent fitness increase in some locations.
By the end of the nineteenth century the melanic form (carbonaria) reached frequencies as high as 98 per cent in parts of Britain. Comparable trends appeared in North America, where up to 90 per cent of moths sampled in certain regions of Michigan and Pennsylvania were melanic in 1959.
Following environmental reforms and improvements in air quality, the pattern reversed. By 2001 the frequency of melanism in the peppered moth population in the same American regions fell to around 6 per cent. These declines correlate with reduced sulphur dioxide pollution, the return of lichens and the increasing survival of pale forms.
Further experimental research in 2018 quantified survival differences under controlled conditions. Light-coloured moths displayed better colour camouflage on lichen-covered bark, whereas melanic forms exhibited improved luminance camouflage against plain bark. Overall, typica moths enjoyed 21 per cent higher survival in clean environments.

Broader Taxonomic Distribution

Industrial melanism is widespread among Lepidoptera, with more than 70 moth species in Britain alone showing melanic forms linked to polluted environments. These include Odontopera bidentata (scalloped hazel), Apamea crenata, Acronicta rumicis and Lymantria monacha (dark arches), many of which display declining melanism as pollution diminishes.
The phenomenon is also documented in other arthropods such as the two-spot ladybird (Adalia bipunctata) and the barklouse Mesopsocus unipunctatus. In vertebrates, industrial melanism has been observed in the turtle-headed sea snake (Emydocephalus annulatus) and may occur in urban populations of feral pigeons.

Controversy and Scientific Debate

Kettlewell’s pioneering studies were broadly accepted but later challenged. Between 1965 and 1969 Theodore David Sargent attempted to replicate his experiments and claimed inconsistencies, suggesting that bird behaviour might have been influenced or misinterpreted. Judith Hooper’s 2002 book Of Moths and Men further alleged flaws in Kettlewell’s methods and motivated critiques from creationist groups.
However, extensive reviews by evolutionary biologists and historians of science—including Laurence M. Cook, Bruce S. Grant, Michael Majerus and David Rudge—have reaffirmed the validity of Kettlewell’s conclusions. Subsequent long-term field studies have consistently demonstrated that the rise and fall of melanism correlate with environmental changes, providing strong empirical support for natural selection acting through differential predation.
Majerus’s later experiments, conducted with more rigorous field protocols, confirmed that predation by birds remains the principal selective force shaping melanism in the peppered moth.

Alternative Interpretations

While camouflage is widely accepted as the primary driver of industrial melanism in insects, additional hypotheses have been proposed to explain its occurrence in certain species or environments:

• Immune Function: The darker phenotype may confer a stronger immune response. Melanin helps encapsulate and neutralise foreign bodies, enhancing survival in polluted environments where chemical stressors are more prevalent.
• Thermoregulation: Melanic forms may absorb heat more efficiently under reduced sunlight, although this explanation is secondary to camouflage in most cases.
• Trace Metal Excretion: In some vertebrates such as the turtle-headed sea snake, the presence of dark pigmentation may help eliminate toxic elements. As the snakes shed their skin frequently, melanic scales bind higher concentrations of pollutants, facilitating more effective detoxification.

These supplementary mechanisms may operate alongside or independently of camouflage, depending on the ecological context.

Industrial Melanism as Evolutionary Evidence

The decline of melanism in multiple species following reductions in pollution constitutes a large-scale natural experiment supporting natural selection. As environmental quality improved in Europe and North America, melanic morphs lost their selective advantage, and pale forms resurged. This consistent pattern across taxa and continents reinforces the interpretation that selective predation, mediated by camouflage, is the most credible explanation for the evolutionary dynamics observed.