In the period between 1900 and 1938 (BD) natural selection is complicated by other forces. Though the color of the dark form gives it an advantage over the light, the new trait is introduced into a system of other traits balanced for the light form; thus the dark form is at first at a considerable physiological disadvantage. In fact, when moths of the dark form were crossed with moths of the light form 50 years ago, the resulting broods were significantly deficient in the dark form. When the same cross is made today, the broods contain more of the dark form than one would expect. The system of hereditary traits has become adjusted to the new trait.
There is evidence that other changes take place during the period BD. Specimens of the peppered moth from old collections indicate that the earliest melanics were not so dark as the modern dark form: they retained some of the white spots of the light form. Today a large proportion of the moths around a city such as Manchester are jet black. Evidently when the early melanics inherited one gene for melanism, the gene was not entirely dominant with respect to the gene for light coloration. As the gene complex adjusted to the mutation, however, the new gene became almost entirely dominant.
When the dark form comprises about 10 percent of the population, it may jump to 90 percent in as little as 15 or 20 years. This is represented by period DE on the graph. Thereafter the proportion of the dark form increases at a greatly reduced rate.
Eventually one of two things must happen: either the light form will slowly be eliminated altogether, or a balance will be struck so that the light form continues to appear as a small but definite proportion of the population. This is due to the fact that the moths which inherit one gene for dark coloration and one for light (heterozygotes) have an advantage over the moths which inherit two genes for dark coloration (homozygotes). And when two heterozygotes mate, a quarter of their offspring will have two genes for light coloration, i.e., they will be light. Only after a very long period of time, therefore, could the light forms (and with them the gene for light coloration) be entirely eliminated. This period of removal, represented bv EF on the diagram, might be more than 1,000 years. Indications so far suggest, however, that complete removal is unlikely, and that a balance of the two forms would probably occur. In this balance the light form would represent about 5 percent of the population.
The mechanisms I have described are without doubt the explanation of industrial melanism: normal mutation followed by natural selection resulting in an insect of different color, physiology and behavior. Industrial melanism involves no new laws of nature; it is governed by the same mechanisms which have brought about the evolution of new species in the past.
There remains, however, one major unsolved problem. Why is it that, in almost all industrial melanics, the gene for melanism is dominant? Many geneticists would agree that dominance is achieved by natural selection, that it is somehow related to a successful mutation in the distant past. With these thoughts in mind I recently turned my attention away from industrial centers and collected moths in one of the few remaining pieces of ancient Caledonian pine forest in Britain: the Black Wood of Rannoch. Located in central Scotland far from industrial centers, the Black Wood is probably very similar to the forests that covered Britain some 4,000 years ago. The huge pines of this forest are only partly covered with lichens. Here I found no fewer than seven species of moths with melanic forms.
I decided to concentrate on the species Cleora repandata, the dark form of which is similar to the dark form of the same species that has swept through central England. This dark form, like the industrial melanics, is inherited as a Mendelian dominant. Of just under 500 specimens of C. mpandata observed, 10 percent were dark.