Henry Bernard Davis Kettlewell’s work with the peppered moth (Biston betularia) illustrates how species can change over time. His mid-1950s experiments provided direct observations related to the theory of evolution by natural selection. Kettlewell aimed to understand how environmental pressures influenced the survival rates of the moth’s different color forms. His findings contributed to understanding how these pressures drive visible evolutionary changes.
The Experiment’s Setup
Kettlewell’s experimental design involved a “mark-release-recapture” technique to compare the survival of light and dark peppered moths. He obtained both the light-colored typica form and the dark-colored carbonaria form. Each moth was marked with a dab of cellulose paint on the underside of its wing, allowing researchers to distinguish them upon recapture.
He chose two distinct woodland sites: a heavily polluted area near Birmingham and an unpolluted, rural area in Dorset. The Birmingham site had trees darkened by industrial soot, while the Dorset site featured lighter, lichen-covered tree trunks. The hypothesis was that moth coloration would influence their camouflage against these backgrounds, affecting their susceptibility to bird predation. Moths were released into these environments on tree trunks or branches.
The Crucial Recapture and Results
After releasing the marked moths, Kettlewell and his team employed various methods to recapture them over subsequent nights. They primarily used mercury vapor light traps and assembling traps, which contained virgin female moths to attract males. This phase aimed to determine the differential survival rates of the moth forms in each environment.
The recapture data revealed clear patterns. In the polluted Birmingham woodland, a significantly higher proportion of dark moths were recovered compared to light moths. For instance, in a 1953 experiment, 27.5% of released dark moths were recaptured, while only 13.1% of light moths were. Conversely, in the unpolluted Dorset site, a greater proportion of light moths were recaptured. These results indicated that moths whose coloration blended with their environment had a better chance of surviving predation by birds.
Demonstrating Natural Selection
Kettlewell’s recapture results provided evidence for natural selection in action. The differential survival rates observed directly correlated with the moths’ camouflage against their respective backgrounds. In polluted areas, dark moths blended with soot-covered trees, making them less visible to predatory birds. Conversely, in unpolluted areas, light moths were better camouflaged against lichen-covered trees.
This experiment demonstrated how environmental pressures, specifically bird predation, could lead to changes in the frequency of different traits within a population. The more camouflaged moths were more likely to survive, reproduce, and pass on their advantageous coloration to their offspring. Kettlewell’s work became a key example of observable evolution, supporting Darwin’s theory of natural selection.