The peppered moth (Biston betularia) is a night-flying moth found across the Northern Hemisphere. Its historical shifts in coloration serve as a widely cited illustration of evolutionary change driven by natural selection.
The Moth Before Industry
Before industrialization, the peppered moth primarily existed as the light morph (typica). Its speckled wings provided effective camouflage, allowing the moths to blend against light, lichen-covered tree bark where they rested during the day.
Hiding from predators, primarily birds, was crucial for survival. Against the mottled background of lichen and bark, light moths were difficult for birds to spot, reducing predation. A less common dark form (carbonaria) also existed due to a natural genetic mutation. These dark moths were highly visible against the light bark and consequently rare, comprising perhaps 0.01% of the total moth population.
The Industrial Revolution’s Impact
The Industrial Revolution in England (18th-19th centuries) dramatically altered the peppered moth’s environment. Coal-fueled factories released immense amounts of soot and sulfur dioxide into the atmosphere. This widespread air pollution had a profound effect on the landscape.
Soot blackened tree trunks and walls, and sulfur dioxide killed light-colored lichens. This compromised the light typica moths’ natural camouflage. Against darkened surfaces, pale moths became visible to predatory birds, creating selective pressure.
Evolution in Action
This drastic environmental change set the stage for a remarkable evolutionary shift. The once rare dark, melanic form (carbonaria), which was previously disadvantaged, now found itself with a survival advantage. Its dark coloration provided superior camouflage against the soot-blackened tree trunks, making it much harder for birds to detect.
Predatory birds, like tits, selectively preyed on the more visible light moths, while the dark moths were more likely to survive and reproduce. This differential predation led to a rapid increase in the frequency of the dark morph within polluted regions. For instance, the first recorded dark peppered moth appeared in Manchester in 1848, and by 1895, the carbonaria form constituted approximately 98% of the peppered moth population in that city.
Scientific observations and experiments provided compelling evidence for this evolutionary response. Bernard Kettlewell’s work in the mid-1950s, for example, involved releasing and recapturing light and dark moths in both polluted and unpolluted woodlands. His studies demonstrated that in polluted areas, a higher proportion of dark moths survived and were recaptured, while in unpolluted areas, light moths had a greater survival rate. These experiments confirmed that birds were indeed the selective agents driving the observed changes in moth populations.
Modern Revival and Lasting Lessons
Following the peak of industrial pollution, environmental regulations, such as the Clean Air Act of 1956 in the UK, led to significant reductions in airborne soot and improved air quality. As a consequence, tree bark began to lighten again, and lichens started to recolonize. This reversal of environmental conditions subsequently led to a reversal in the evolutionary trend of the peppered moth.
With the return of lighter, lichen-covered trees, the light-colored typica moths once again gained their camouflage advantage. The dark carbonaria form became more conspicuous, making them more vulnerable to bird predation. This shift resulted in a decrease in the frequency of dark moths and a resurgence of the light morph in many areas, a process that has been observed in various regions across Europe and the USA.
The peppered moth’s story stands as a powerful and observable example of natural selection in action, demonstrating how populations can adapt to changing environmental pressures. It remains a classic case study in science education, illustrating fundamental evolutionary principles such as adaptation, genetic variation, and the role of selective pressures in shaping species over time.