A Lesson in Natural Selection: Evolution of the Peppered Moth

A brief overview of evolution makes it clear that evolutionary changes occur over long periods of time, often millenia. However, human advances today take as little as a few years. Are humans the only organisms like this: undergoing rapid changes that don’t follow evolutionary trends? Evolutionary biology shows that humans are not the only outliers.

Natural selection of the peppered moth during the Industrial Revolution in 19th-century England is an example of an outlier in the study of evolution. During this time, pollution from factories filled forests, coating trees in a layer of soot—turning white trees black.

This change in the color of the trees led to rapid changes in peppered moth populations in the region: within a few years, the less common, dark moths dominated, while the more common white ones came close to extinction.

The evolution of the peppered moths is an example of rapid evolutionary selection that occurred under atypical circumstances, much like the strange idea of human evolution not keeping up with human progress.

Highlighted below is an in-depth explanation of the evolution of the peppered moth, which provides a better understanding of the implications of rapid human progress.

The peppered moth comes in primarily two varieties: typica, a white moth with a sprinkling of black dots (wild type), and carbonaria, almost uniform black moths (mutant).

Usually, the white peppered moths are selected for because they blend in with the white trees on which they live, while the uniform black moths are selected against because their lack of camouflage leaves them vulnerable to bird predation. ¹

However, at the height of the Industrial Revolution, soot from the factories covered the trees as a result of coal pollution, transforming the white lichen-covered trees to black ones. At this point, the carbonia type sharply increased in prevalence because of its ability to camouflage against the now-black trees. This increase was so great, in fact, that the wild type variety of the peppered moth became almost extinct by the end of the century.

These two moth varieties became the most widely-known example of industrial melanism, the prevalence of dark-colored varieties of animals in industrial areas where they are better camouflaged against predators than paler forms. ²

After environmental regulations later in the Industrial Revolution, the amount of soot expelled into the environment decreased. The trees became white again, and the evolutionary advantage shifted back to the white peppered moths. The carbonaria form is now rare across Britain.

The study of the peppered moths was one of the earliest tests of Charles Darwin’s theory of natural selection, the process by which organisms better-adapted to their environment survive and produce more offspring than their counterparts.

This theory is comprised of four main postulates: individuals in a population vary, at least some variation is heritable, some individuals leave more offspring than others, and the variation in survival and reproduction is not random but rather depends on heritable trait variation. Basically, genetic variants with higher survival or reproduction increase in frequency in a population. ³

Though these findings about peppered moths filled an important gap in the study of microevolutionary change, the exact genetic mechanism responsible for industrial melanism was not identified until much later.