The birch moth (Biston betularia) is a small nocturnal butterfly widespread throughout Europe and North America. Although not particularly striking, it is well known because it appears in all school books as an example of evolution by natural selection : the two forms of the butterfly (white and black) have a different survival depending on the environment in which they live, a fact that is the basis of evolution.

A group of British researchers has now discovered which part of the genome changes the color of the butterfly.

See also:
10 things you think you know about evolution

A textbook example. The history of research on this birch butterfly dates back to the 1950s, when an English naturalist, Bernard Kettlewell, studied the evolutionary dynamics of the two forms of this moth (the white one, called typica, and the black one, carbonaria). His research, which lasted a few years, was aimed at clarifying why in some areas the clear butterfly was much more widespread than the black one, while the opposite occurred elsewhere.

Thus he discovered that, under natural conditions, the first was camouflaged on the trunks of trees covered with white lichens, while the second - black - on the same background was clearly visible and therefore easy prey for birds. Kettlewell made it clear that the black form was more frequent than the white one in the polluted areas.

In the nineteenth century, in fact, the Industrial Revolution had in some parts of England killed lichens and covered the trunks of soot. Right here dark butterflies survived more because they were less visible to predators.

The naturalist then demonstrated as an agent of natural selection (insectivorous birds) could quickly change the appearance of an animal species and then evolve it.

Walking genes. However, it was not clear how the "jump" took place from the white to the black form. Even knowing that he had a genetic basis, evolutionists had not yet figured out which genes were responsible. Some English scholars have succeeded in discovering the element that causes this change, and have published the work in the journal Nature. According to this research, the mutation involves about 22, 000 nucleotides (the single bricks of the DNA) that are inserted in a tract of the gene called cortex, which in turn influences the formation of the butterfly's wings.

This fragment of genetic material is part of the so-called transposons, traits of DNA capable of moving from one position to another of the genome, thus changing (sometimes) the metabolism or the appearance of the body. Not only: by computer simulating the probability of the displacement of this transposon, the scientists were able to determine that the mutation appeared for the first time around 1819. A date that is well combined with the first observations of dark butterflies, which date back to the middle of the forties of two centuries ago.