Dr. Bernard Kettlewell, an entomologist, conducted pioneering field research in the mid-1950s using the peppered moth (Biston betularia) to investigate the theory of natural selection. His work sought to answer how well different individuals survived in their natural environment. The study provided some of the first direct, experimental proof that environmental changes could rapidly drive a shift in a species’ observable traits. Kettlewell needed a robust method to measure the survival rate of one moth color variant against another.
The Evolutionary Context of Industrial Melanism
The peppered moth population in Great Britain historically consisted mostly of a light, mottled form known as typica, which blended well with lichen-covered tree trunks. The Industrial Revolution brought heavy pollution, especially in manufacturing centers like Birmingham, which darkened the landscape. This environmental shift changed the selective pressure acting on the moth population.
The soot and smoke killed the pale lichens and blackened the bark of trees, creating a dark background. Against this newly darkened backdrop, a rare, naturally occurring dark form of the moth, known as carbonaria or the melanic morph, suddenly gained a camouflage advantage. The light-colored moths, once well-hidden, became highly conspicuous targets for predators.
Kettlewell’s Experimental Design: Mark-Release-Recapture
Kettlewell’s method for determining which moths survived longer was based on the Mark-Release-Recapture (M-R-R) technique. This technique allowed him to track the fate of specific individuals of both color types in contrasting environments. He chose two main locations for his field trials: a heavily polluted woodland near Birmingham and a clean, unpolluted area in Dorset, which still had lichen-covered trees.
Before release, every moth was marked with a tiny, inconspicuous dot of paint, usually on the underside of a wing. This mark served as a unique identifier, allowing him to distinguish an experimental moth from a wild one upon recapture.
Kettlewell then released hundreds of marked light and dark moths into the two distinct woodlands. For example, in the polluted Birmingham woods, he released 630 moths, and in the unpolluted Dorset woods, he released 984 moths in one trial. Over the following nights, Kettlewell and his team used light traps and assembling cages to recapture the released individuals. The proportion of marked moths recaptured would be a reliable proxy for their survival rate.
Calculating Differential Survival and Predation
The ratio of recaptured moths served as a direct measure of differential survival against predation. The number of moths of each color morph recaptured, compared to the number released, allowed Kettlewell to infer which type had a better chance of avoiding predators. A low recapture rate suggested that a greater number of those individuals had been removed from the population, primarily through bird predation.
In the sooty woodlands of Birmingham, Kettlewell found that the dark carbonaria morph was recaptured at a significantly higher rate than the light typica morph. For instance, the recapture rate for the dark form was 27.5%, while the light form’s rate was only 13%. This difference indicated that the light moths were being selectively preyed upon because they were easily seen against the dark tree bark. Conversely, in the clean, lichen-rich woods of Dorset, the result was reversed: the light moths had a much higher recapture rate (13.7%) than the dark moths (4.7%). This clear correlation between moth coloration, environmental background, and recapture rates provided strong evidence that birds were the selective agents removing the more conspicuous moths.
The Legacy of the Peppered Moth Study
Kettlewell’s field experiments provided quantitative data supporting Charles Darwin’s theory of natural selection. By demonstrating that the survival advantage of the light and dark moths was dependent on the environment, he offered an observed example of evolution in action over a relatively short period. The study became one of the most famous examples of natural selection in the biological sciences.
The legacy of the peppered moth study was reinforced by a reverse experiment that unfolded decades later. Following the introduction of Clean Air Acts in the 1950s and 1960s, which significantly reduced industrial soot pollution, the environment began to clean up. As the lichens returned to the trees, the selective pressure shifted back toward the lighter morph. Subsequent research observed a corresponding decline in the frequency of the dark carbonaria form and a resurgence of the light typica form, further confirming the direct link between environmental change, camouflage, and differential survival.