The Peppered Moth Story

In the early 1800s, a small black moth was found in Manchester for the first time. The moth was however not a new species, but an unusual black variety of the Peppered Moth, an insect more commonly seen with  pale grey with black speckles. The new black variety, named ‘carbonaria’, was found more and more through the century in the industrialised north of England. By the end of the century they had more or less replaced the original paler moth.

© Olei Leillinger

From the 1950s, scientists hypothesised that the pale Peppered Moth had been replaced by the darker form due to birds eating the paler version. The pale moth’s speckling makes it well camouflaged among lichen on the bark of trees where it rests during the day, but the black moths looked better camouflaged on the soot-covered bark of trees around smoky cities. Experiments with moths placed on tree trunks showed that birds did find moths based on the colour of the bark of trees, and the story became a classic example of evolution by natural selection.

The story of the Peppered Moth is a powerful one as it is so easy to understand. Dark moths against dark trees are better camouflaged than the pale moths, and the birds eat moths based on which ones are easiest to find. The colour of the moth is genetically determined, and therefore this is the definition of evolution, namely change in gene frequency over time. As the change is due to different levels of predation on one colour variety, this is natural selection. Interestingly, after the Clean Air Acts of the 1950s, dark moths declined and were subsequently replaced by the ‘original’ pale moths, which were better camouflaged on the bark of trees no longer covered by soot.