How could the first dark-coloured moth have occurred in the population?

The light-colored form of the moth was predominant in England before the industrial revolution. In the mid-nineteenth century, a dark-colored form appeared. The difference is produced by a dominant allele of one gene. By about 1900, approximately 90% of the moths around industrial areas were dark colored, whereas light-colored moths were still abundant elsewhere. Apparently, birds could readily find the light moths against the soot-darkened background in industrial areas and therefore were eating more light moths.

Nature selected the dark-coloured moth over the light because only the dark could withstand the societal pressures after the industrial revolution, as they were most likely to blend in and not be seen by their natural predators. The light-coloured moths, however, were more easily seen and their numbers decreased. This is a form of microevolution, more specifically genetic drift, which is a change in the gene pool of a population due to chance. In other words, it is partly chance that affects which moths reproduce.

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