Melanic forms of many species of moth are independent of industrialization. The factors that can favor melanism are numerous and varied. These have been discussed in detail by Kettlewell and Majerus. Their relevance to industrial melanism is that in some moths, the presence of melanic forms prior to, and independent of, industrialization provided a repository of melanic variants that were favored as pollution levels increased.
Indeed, it is likely that the majority of moths that exhibit melanic polymorphism, with melanic frequency correlated to pollution levels, had melanic forms occurring at relatively low equilibrium frequencies prior to the industrial revolution. Changes in the environment resulting from increased pollution favored these dark forms and their frequencies increased. The willow beauty, Peribatodes rhomboidaria, illustrates the idea well. In Britain, this species has long had a nonindustrial melanic form, perfumaria. The perfumaria form greatly increased in frequency in industrial regions in the late 19th century. In the 20th century, perfumaria, which still occurs at low frequency in some rural areas, particularly in Scotland, was replaced in industrial areas, but not elsewhere, by an even darker form, f. rebeli. Here then, f. perfumaria should be regarded as a partial industrial melanic, while f. rebeli is a full industrial melanic.
Many probable instances of partial industrial melanic polymorphism could be cited, but rather few of the species that fall into this class of melanism have been investigated in any depth. Exceptions are the pale brindled beauty, Phigalia pilosaria; the mottled beauty, Alcis repandata; and the green brindled crescent, Allophyes oxyacanthae. All are trunk-resters, the increase in melanics in industrial regions being attributed to increased crypsis.
Some of these species show morph-specific habitat preferences. Morph-specific habitat preferences in Lepidoptera showing melanic polymorphism were first suggested to explain abrupt differences in melanic frequencies of the mottled beauty and the tawny-barred angle, Semiothisa liturata, either side of sharp habitat boundaries. Such differences have subsequently been recorded in 14 species, in all cases melanics having higher frequencies in woodland with dense canopies than in adjacent more open habitats.
Many species that now have industrial melanics first evolved melanism in specific ecological circumstances prior to industrialization. It is known, for example, that a number of species now show melanic polymorphism in unpolluted ancient coniferous forests, such as Rannoch Black Wood in Scotland. Similar habitats were more widespread in the past and are likely to have supported melanic forms. These melanic forms would have been at a selective disadvantage if they moved from areas with the specific ecological circumstances to which they were adapted. Consequently, the melanics evolved behaviors that restricted them to such habitats. Recent changes in forestry and land usage and increases in pollution have provided new habitats (e.g., conifer plantations, polluted woodlands) with ecological conditions that favor melanics. The melanics have consequently spread and risen in frequency, producing examples of partial industrial melanic polymorphism in which morph-specific habitat preferences are retained to some extent.
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