For the first time, moreover, we had witnessed birds in the act of taking moths from the trunks. Although Britain has more ornithologists and bird watchers than any other country, there had been absolutely no record of birds actually capturing resting moths. Indeed, many ornithologists doubted that this happened on any large scale.
The reason for the oversight soon became obvious. The bird usually seizes the insect and carries it away so rapidly that the observer sees nothing unless he is keeping a constant watch on the insect. This is just what we were doing in the course of some of our experiments. When I first published our findings, the editor of a certain journal was sufficient- 1y rash as to question whether birds took resting moths at all. There was only one thing to do, and in 1955 Niko Tinbergen of the University of Oxford RImed a repeat of my experiments. The RIm not only shows that birds capture and eat resting moths, but also that they do so selectively.
These experiments lead to the following conclusions. First, when the environment of a moth such as Biston betularia changes so that the moth cannot hide by day, the moth is ruthlessly eliminated by predators unless it mutates to a form that is better suited to its new environment. Second, we now have visible proof that, once a mutation has occUlTed, natural selection alone can be responsible for its rapid spread. Third, the very fact that one form of moth has replaced another in a comparatively short span of years indicates that this evolutionary mechanism is remarkably flexible.
The present status of the peppered moth is shown in the map on the opposite page. This map was built up from more than 20,000 observations made by 170 voluntary observers living in various parts of Britain. The map makes the fol· lowing points. First, there is a strong correlation between industrial centers and a high percentage of the dark form of the moth. Second, populations consisting entirely of the light form are found today only in western England and northern Scotland. Third, though the counties of eastern England are far removed from industrial centers, a surprisingly high percentage of the dark form is found in them. This, in my opinion, is due to the long-standing fallout of smoke particles carried from central England by the prevailing southwesterly winds.
Now in order for the dark form of a moth to spread, a mutation from the light form must first occur. It appears that the frequency with which this happens- that is, the mutation rate-varies according to the species. The rate at which the light form of the peppered moth mutates to the dark form seems to be fairly high; the rate at which the mutation occurs in other species may be very low. For example, the light form of the moth Procus literosa disappeared h'om the Sheffield area many years ago, but it has now reappeared in its dark form. It would seem that a belated mutation has permitted the species to regain lost territory. Another significant example is provided by the moth Tethea ocularis. Prior to 1947 the dark form of this species was unknown in England. In that year, however, many specimens of the dark form were for the first time collected in various parts of Britain; in some districts today the dark form now comprises more than 50 percent of the species. There is little doubt that this melanic arrived in Britain not by muta· tion but by migration. It had been known for a considerable time in the industrial areas of northern Europe, where presumably the original mutation occurred.
The mutation that is responsible for industrial melanism in moths is in the majority of cases controlled by a single gene. A moth, like any other organism that reproduces sexually, has two genes for each of its hereditary characteristics: one gene from each parent. The mutant gene of a melanic moth is inherited as a Mendelian dominant; that is, the effect of the mutant gene is expressed and the effect of the other gene in the pair is not. Thus a moth that inherits the mutant gene from only one of its parents is melanic.