Evolution in Peppered Moths

Since Darwin proposed the theory of evolution, a vast literature has emerged that supports the precepts of the theory and increases our understanding of the mechanisms of evolution. One of the most striking cases of evolution in action is that of industrial melanism, or darkening of the body’s tissues, in the peppered moth of England. Early in the nineteenth century, occasional dark (melanistic) specimens of the common peppered moth were collected. Over the next 100 years, this darker form, referred to as carbonaria, became increasingly common in forests near heavily industrialized regions of England, which is why the phenomenon is often referred to as industrial melanism. In places without factories and other heavy industry, the lighter-colored, salt-and-pepper form of the moth prevailed. This phenomenon aroused considerable interest among geneticists, who showed by cross-mating light and dark forms that melanism is an inherited trait determined by a single dominant gene. Because the melanistic trait is an inherited characteristic, its spread reflected genetic changes (evolution) in the population.

Peppered moths inhabit dense woods and rest on tree trunks during the day. Where melanistic individuals had become common, the environment must somehow have been altered so as to give dark forms a survival advantage over light forms. It seemed reasonable to suppose that natural selection-the process by which organisms best suited to survival in their environment achieve greater reproductive success, allowing advantageous genetic characteristics to pass on to future generations-had led to the replacement of the typical light individuals with darker carbonaria individuals. To test this hypothesis, the English biologist H. B. D. Kettlewell captured a sample of moths of both forms, marked each individual with a dot of cellulose, and then released them back into the woods. The mark was placed on the underside of the wing so that it would not attract the attention of predators to a moth resting on a tree trunk. Some days later Kettlewell captured more moths by attracting them to a mercury-vapor lamp in the center of the woods or to caged virgin females at the edge of the woods. (Only males could be used in the study because females are attracted neither to lights nor to virgin females.)This type of study is referred to as a mark-recapture study.

In one such mark-recapture experiment, Kettlewell marked and released 201 lighter moths and 601 darker moths in a wooded area near industrial Birmingham, England. The results indicated that a greater percentage of the darker form survived over the course of the experiment. A similar experiment in a nonindustrial area revealed higher survival by the lighter salt-and-pepper form of the moth.

The specific agent of selection was easily identified. Kettlewell reasoned that in industrial areas pollution had darkened the trunks of trees so much that typical moths stood out against them and were readily found by predators. Any unusual dark forms were better camouflaged against darkened tree trunks, and their coloration made them more likely to survive. Eventually, the different survival rates of dark and light forms would lead to changes in their relative frequency in the population. To test this idea, Kettlewell placed equal numbers of light and dark forms on tree trunks in polluted and unpolluted woods and watched them carefully from a concealed spot some distance away. He quickly discovered that several species of birds regularly searched tree trunks for moths and other insects, and that these birds more readily found a moth that contrasted with its background than one that resembled the bark it clung to. These data were consistent with the results of the mark-recapture experiments. Together they clearly demonstrated the operation of natural selection, which over a long period resulted in genetic changes in populations of the peppered moth in polluted areas.

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