Industrial Melanism in the Peppered Moth
Perhaps the most famous example of evolution in action is the case of the peppered moth (
Biston betularia), which illustrates the concept of “selection pressure” – the force that drives evolution.
The black form of the peppered moth thrived in polluted industrial areas. Photograph by L.B. Tettenborn
The British peppered moth feeds at night and spends the day resting on tree trunks, where it is at risk of being eaten by birds. Until the mid-1890s, all peppered moths had a pale, speckled coloration that provided
camouflage against the pale lichen that covered the bark of their trees. In the latter half of the 19th century, however, a black form of the moth was first observed, and by 1900 almost all peppered moths in urban areas were black, whereas most of those in rural areas remained pale.
This 50-year period coincided with the rise of industrialization in British cities, and it was suggested that the change in moth color in urban areas was due to sulfur dioxide fumes from factories killing the lichen on tree trunks. The dark bark beneath was good camouflage for black peppered moths, but pale moths were now at a disadvantage because they were more easily spotted by predatory birds.
In 1956, the entomologist H. Bernard Kettlewell set about examining this hypothesis by collecting, marking and releasing both black and pale peppered moths in Birmingham (an urban area) and Dorset (a rural area). He then laid more traps to recapture the marked moths, and observed that a smaller proportion of the pale form was recaptured in Birmingham and a smaller proportion of the dark form in Dorset.
This finding was consistent with the suggestion that pale moths were more conspicuous – and therefore easier prey for birds – in areas with dark, polluted trees, whereas dark moths were more likely to be eaten in rural areas with pale, lichen-covered trees. It could be concluded that the selection pressure of bird predation had driven the evolution of two different forms of B. betularia.
Kettlewell’s experiment has been
repeated, and his conclusion confirmed, several times, most recently in 2003. Since the Clean Air Act of 1956, however, emissions of sulfur dioxide have decreased in the UK, and these later studies have documented the decline of the dark peppered moth in industrial regions. By 1985, it had largely disappeared from the Midlands and was found in high numbers only in the far North East of England.
