This species of moth, the Bistun betularia, spends its day resting on the bark of trees or rocks. Before the industrial revolution, when the birch trees were clear and lichens on the bark very often, specimens of this moth, white and spotted with black, were very frequent and almost invisible as aided by bark similar to the color of the wings of the moth, which constituted a significant protection against attack by predators.
When, due to strong industrialization, the lichens died and the bark of trees and rocks were darkened by soot, the Biston betularia few specimens were hunted as predators on the trees become a source of danger to the moths, only a few specimens survived more clear.
Thus, since 1848, the first moths were found dark, almost invisible on a foggy bottom and at the end of the nineteenth the dark varieties accounted for about 90% of the population these moths.
The two forms, the clear and dark, were always present: the gene for light-colored suit to the colored form with a low frequency, which is sufficient, however, when this mutation, compared with the white form, a high value selective.
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