The decline of the dark moth population over the last half-century represents a dramatic instance of evolution in reverse. The subject is the Peppered Moth, Biston betularia, which became the textbook example of rapid adaptation. During the Industrial Revolution, a black variant of this moth rapidly rose to dominance in polluted areas, a phenomenon known as industrial melanism. This demonstrated how quickly natural selection acts in response to human-induced environmental change. The dark form, once successful, has been receding in number, making its original, lighter counterpart common again, providing a clear answer to how evolutionary pressures change over time.
The Conditions That Created Dark Moths
The prevalence of the dark-colored moth was a direct consequence of widespread industrial activity across Britain and other industrialized nations in the 19th century. Burning massive quantities of coal released enormous amounts of soot and sulfur dioxide into the atmosphere. This dense pollution blanketed the landscape, fundamentally altering the natural camouflage of the moths’ resting spots.
Sulfur dioxide emissions proved toxic to the light-colored lichens that typically grew on tree bark, causing them to die off near industrial centers. Without the speckled, pale lichens, tree trunks were left exposed and blackened by soot. This environmental transformation instantly shifted the selective advantage away from the common, speckled light-colored moth.
The rare dark variant, known as carbonaria, possesses a mutation causing almost completely black wings, making it virtually invisible against the sooty bark. Birds, the primary predators, easily spotted and consumed the light-colored individuals resting on the darkened trunks. The dark moths had a higher survival rate, meaning they were more likely to reproduce and pass their gene to the next generation, driving the frequency of the dark form to nearly 98% in some areas by the late 1800s.
The Reversal of Selection Pressure
The dark moth’s subsequent decline resulted from major policy changes that reversed the pollution creating their selective advantage. Beginning in the mid-20th century, anti-pollution legislation, such as the UK’s Clean Air Act of 1956, significantly improved air quality across industrial regions. This legislation controlled the emission of smoke and sulfur dioxide, dramatically reducing the soot coating the environment.
The cleaner air allowed sensitive organisms like lichens to recover and recolonize the tree trunks and branches. As the lichens regrew, the tree bark returned to its natural, mottled, lighter coloration, restoring the environment to its pre-industrial state. The light-colored, speckled moth, known as typica, once again blended seamlessly into the background.
This environmental cleanup reversed the camouflage advantage the dark carbonaria moth had enjoyed for over a century. The dark moths now stood out conspicuously against the pale, lichen-covered bark, making them easier targets for predatory birds. Consequently, the light-colored moths experienced a strong selective advantage, surviving long enough to breed and rapidly increasing their frequency. Near Liverpool, the proportion of dark moths fell from over 90% in 1959 to just 61% by 1984, demonstrating the swiftness of this evolutionary reversal.
Scientific Methods Used to Track the Shift
The decline of the dark moth population has been documented by scientists using long-term ecological and experimental methods. The primary method involves large-scale monitoring using light traps to capture moths. By counting the different color forms over decades and across various regions, researchers established a precise correlation between falling air pollution levels and the decreasing frequency of the dark carbonaria variant. The data collected since the 1960s offer a clear timeline of the reversal.
A crucial part of confirming the cause was the use of predation experiments, pioneered by Bernard Kettlewell in the 1950s. These experiments involved releasing equal numbers of light and dark moths into different types of woodland—polluted (sooty) and unpolluted (lichen-covered)—and recording the recapture rates. In unpolluted areas, the light form was recaptured at a higher rate, indicating better survival, while the reverse was true in polluted woods.
Modern researchers validated these findings using artificial moth targets and digital imaging analysis to quantify camouflage from a bird’s perspective. For instance, a recent study used artificial targets pinned onto lichen-covered trees, demonstrating that pale moths had a 21% greater survival advantage against bird predation. These experiments provided direct evidence that differential bird predation, driven by background color matching, is the major factor driving the change in gene frequency.
Contemporary Threats to Moth Populations
While the decline of the dark form is a specific case of natural selection reversing its course, the overall population of the Peppered Moth and many other moth species now face entirely different, non-selective threats. These modern pressures affect all color forms equally and represent a new conservation challenge distinct from the classic evolutionary story of industrial melanism.
Habitat Fragmentation
Habitat fragmentation poses a significant challenge, as the division of natural areas by human development limits the moth’s movement and access to host plants for their larvae. The destruction of forested and natural lands to make way for intensive agriculture and urban expansion removes the essential resources the moths rely on for their life cycle.
Chemical Pesticides
The widespread use of chemical pesticides in both agricultural and suburban environments is a major contributor to general moth decline. These chemicals, designed to eliminate pests, often impact non-target insects, leading to population crashes across numerous species.
Artificial Light at Night (ALAN)
Artificial light at night (ALAN), commonly known as light pollution, disrupts the nocturnal behavior of moths, which are primarily active after dark. Studies have shown that nighttime lighting can disorient moths, making them more vulnerable to predators and disrupting their ability to forage, mate, and reproduce. In some lit areas, caterpillar populations have been found to be nearly 50% lower than in unlit areas. This indicates that light pollution is a significant driver of overall insect decline, regardless of the individual moth’s color.