This article is from the magazine Les Indispensables de Sciences et Avenir n°208 of January/March 2022.
When a butterfly reveals its dark side
The birch moth (Biston betularia) is considered the emblematic example of evolution in action. This small butterfly usually has light-colored wings decorated with darker spots that form a mosaic (typical shape). But in 1848, an entirely black form of the insect was discovered in England near Manchester, where it became increasingly abundant over time. We are then in the midst of the industrial revolution, and the factories in the north of the country are spitting out their coal soot in a continuous stream. In 1896, a few years after the publication of Darwin’s theories, a naturalist committed to evolutionary ideas, James William Tutt, argued that the dark forms – called carbonaria – were favored because they went unnoticed over logs blackened by pollution. .thus escaping the sight of birds of prey.
In 1955, the British zoologist Bernard Kettlewell developed an experiment to test this hypothesis of natural selection. He proceeds to release light and black moths, in one contaminated and one uncontaminated area, before recapturing them and counting the survivors. It seems that the black form is much more conserved in polluted regions, while it is the preferred target of birds in uncontaminated areas, where its dark color contrasts with the light trunks. A perfect example of selective advantage to illustrate the theory of evolution.
Too perfect… Because the Kettlewell experiment raises methodological criticism. It is criticized for numerous biases, in particular for having used very few insects and for having unrealistically placed them on the trunks (this would not be the main natural refuge of the moth). For many creationists, the little butterfly becomes the shining symbol that the theory of evolution is a farce! However, in the 1990s, the British biologist Michael Majerus showed that Kettlewell’s conclusions should not be disqualified.
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