Three categories of industrial melanism have been recognized:
A. Full industrial melanic polymorphism involves distinct melanic forms that have arisen since the industrial revolution and have increased as a consequence of the effects of industrialization on the environment.
B. Partial industrial melanic polymorphism involves polymorphic species that had melanic forms prior to the industrial revolution. These forms have increased in frequency following and as a consequence of the effects of industrialization.
C. Polygenic industrial melanism involves species in which the average ground color of some or all members of a population has darkened gradually as a consequence of the effects of industrialization.
It should be noted that melanism is a common phenomenon throughout the animal kingdom, with many factors unrelated to industrialization or pollution influencing the success of melanic forms in some species.
FULL INDUSTRIAL MELANIC POLYMORPHISM The Peppered Moth
The peppered moth, Biston betularia, has dominated the literature on industrial melanism. In Britain, the ancestral form of this species (form typica) is white, liberally speckled with dark brown or black scales (Fig. 1). In 1848, a predominantly black form of B. betularia, form carbonaria (Fig. 2) was recorded in Manchester, England. Within 50 years, 98% of Mancunian peppered moths were black. From this original location, carbonaria spread to many other parts of Britain. The renowned Victorian lepidopterist J. W. Tutt was the first to suggest that camouflage and bird predation could be involved in the spread of carbonaria. In 1896, arguing that the typical form was camouflaged well on surfaces covered by foliose lichens, he noted that the nature of many natural surfaces had changed as a consequence of pollutants resulting from heavy industry. In particular, the combined effects of sulfur dioxide, which killed foliose lichens, and soot fallout, which blackened the denuded surfaces, had led to darker and more uniform substrates. He stated that on these surfaces the carbonaria form would be better camouflaged than typica and so gain protection from bird predation. Natural selection, through the medium of differential bird predation, augmented by "hereditary tendency," had led to an increase in the frequency of the black form.
Tutt's hypothesis was largely rejected, both at the time and for a considerable period thereafter, because most entomologists and ornithologists concurred in the view that birds are not major predators of cryptic, day-resting moths. A variety of other explanations of the increase in melanic forms of some moths were thus put forward during the first half of the 20th century (pollutants acting as mutagenic agents, Lamarckian evolution, heterozygote advantage).
One important advance during this period was the calculation by Haldane in 1924 that carbonaria would have to have been one and a half times as fit as typica to account for the rapidity of the rise in carbonaria frequency in Manchester. This fitness difference was much higher than most evolutionary biologists of the time thought feasible.
Not until the 1950s was Tutt's bird predation explanation of the rise of carbonaria in polluted regions tested by scientific experimentation. Dr. Bernard Kettlewell, using direct observation of the predation of live moths released onto tree trunks, and mark—release—recapture techniques, in two populations, one in a polluted and the other in a nonpolluted oak woodland, obtained strong evidence to support Tutt's
differential predation hypothesis. Both experiments showed that the typica form of the moth had lower fitness than carbonaria in the polluted woodland, but a higher fitness in the nonpolluted wood. It was the fact that Kettlewell obtained reciprocal results in the two environments that made his conclusions so convincing. Kettlewell also mapped the frequency of carbonaria against sulfur dioxide and soot fallout, finding a significant correlation between the frequency of carbonaria and both pollutants, that with sulfur dioxide being strongest. This correlation between high melanic frequencies and high levels of pollutants has been reinforced by the finding that carbonaria frequencies have declined following decreases in pollution levels as a result of anti-pollution and smoke control legislation.
The elements of the basic story of the peppered moth that are usually related are therefore:
1. The peppered moth has two distinct forms.
2. These forms are genetically controlled.
3. Peppered moths rest by day on tree trunks.
4. Birds find peppered moths on tree trunks and eat them.
5. The likelihood of a moth being found by a bird depends on its degree of camouflage.
6. Nonmelanic peppered moths are better camouflaged than melanics on lichen-covered tree trunks in rural areas. Melanic peppered moths are better camouflaged than nonmelanics in industrial areas where tree trunks have been denuded of lichens and blackened by soot fallout.
7. The frequencies of melanic and nonmelanic moths in a particular area depend on the level of bird predation of each form and the rate of migration of moths into the area from adjacent districts in which the form frequencies are different.
Since Kettlewell's research, other studies on the peppered moth, which have included work on the intermediate form, insularia (Fig. 3), and the natural resting behavior of the moth (Fig. 4), have refined some of the details of the case. However, Kettlewell's basic qualitative deductions remain valid.
Caldy Common, West Kirby, northwest England
West Cambridge, England
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