Melanism exemplified in British moths
Melanin is a complex of dark (black, brown, yellowish or dull red) animal pigments, and ‘melanism’ in the present context is the occurrence in a species of some individuals that are predominantly black, dusky or noticeably darker than the typical form, due to a heritable increase in the proportions of melanins in the epidermis (cf. Kettlewell, 1973). Numerous moth species exhibit conspicuous melanism. In some it is known only as a very rare, recessive mutation, but others include populations in which melanic individuals are common or even replace the normal form in certain habitats and/or geographical locations. Inheritance of persistent melanism in moths is sometimes genetically simple, involving two alleles of a single gene with melanism dominant, or resulting in heterozygotes that are more or less intermediate. In other cases, more than two alleles may be involved at the same locus; and in still others, the phenomenon is multifactorial, with alleles at different loci operating cumulatively and resulting in complicated patterns of inheritance leading to populations comprising extremes and intermediates. Melanism in the most intensively studied species, the Peppered Moth (Biston betularia), principally seems to involve five alleles at one locus, of which the three responsible for the insularia (intermediate) complex of forms are dominant over the typica (normal) allele, while the carbonaria (fully melanic) allele is dominant over all of them; and the situation may be further complicated by the operation of modifier genes at other loci. In view of the complicated genetic background here and in the other cases that have been thoroughly investigated, and since relatively few moth species have been genetically detailed in this context, it will usually be impossible to assign field collections (phenotypes) to genotype with any confidence.
From an evolutionary standpoint, persistence of melanic populations in relatively natural habitats (‘rural’ or ‘non-industrial’ melanism) is sometimes reasonably interpretable as resulting from Darwinian natural selective responses, for example to low light intensity in cool and cloudy regions or wet habitats, but few such cases have been thoroughly investigated. However, spectacular changes in the proportions of melanic versus ‘normal’ individuals in the populations of some moth species have been observed, quantitatively detailed and scientifically investigated in Britain for over a hundred years. Being rather obviously correlated with habitat pollution consequent on industrialisation, this phenomenon has long been termed ‘industrial melanism’; and in the case of the Peppered Moth (Biston betularia) in particular, a causal relationship involving Darwinian selection has been quite convincingly demonstrated down the years via numerous, laborious experiments. Researchers’ disagreements over experimental design and interpretation of results, against the complex background of population genetics and ecology, have been misinterpreted by religious extremists unable to grasp that the reality of organic evolution is impervious to arguments over the precise mechanisms. The essential truth has been apparent to informed thinkers for more than two centuries (cf. Lamarck, 1801), and neither it nor the truism of Darwinian natural selection (that the fittest to survive will survive), have ever depended upon support from the Peppered Moth and ‘industrial melanism’. In any case, the basic principles are more readily exemplified nowadays using micro-organisms and viruses, spectacularly so with reference to antibiotic resistance and new strains of influenza.
The accompanying References include detailed, absorbing discussions of melanism and evolutionary interpretations by Ford (1955) and Majerus (1998, 2002). For the present purpose we use the expressions ‘rural melanism’ when it is associated with environmental factors other than pollution (i.e., non-industrial melanism), and ‘industrial melanism’ when observed evolutionary change seems persuasively linked to pollution caused by human activity. Thus broadly interpreted, industrial melanism may be manifested in moth populations either as a conspicuous darkening of the average ground colour, or by marked increase in the proportions of dark individuals (sometimes to the complete elimination of lighter, formerly normal forms), reflecting differences in the underlying genetic backgounds. Illustrations exemplifying melanism in the 28 species of British moths from 8 families in the following list are provided here, with the melanics classified largely with reference to Ford and Majerus. The list includes a few species in which melanism seems to have been noted only since the industrial revolution (‘fully industrial’ melanism), and many more (‘rural to industrial’) where there has been a more or less obvious drift to higher incidences of melanism in species where it had formerly been observed only rarely. Detailed descriptions, illustrations and interactive keys for identifying the families and genera concerned are available in the accompanying ‘British Insects’ packages; viz., ‘The families of Lepidoptera’, ‘The genera of Geometridae’, and ‘The genera of Noctuidae’.
Arctiidae: Diaphora mendica (Muslin Moth: rural, with sex-linked melanism predominant in males); Spilosoma lubricepida (White Ermine: rural); Spilosoma lutea (Buff Ermine: rural). Crambidae: Crambus perlella (rural?). Geometridae: Agriopis leucophaearia (Spring Usher: rural?); Agriopis marginaria (Dotted Border: rural?); Alcis repandata (Mottled Beauty: rural to industrial); Angerona prunaria (Orange Moth: rural); Biston betularia (Peppered Moth: fully industrial); Ectropis crepuscularia (Engrailed: rural to industrial); Epirrita dilutata (November Moth: fully industrial); Erannis defoliaria (Mottled Umber: rural); Odontopera bidentata (Scalloped Hazel: fully industrial); Peribatodes rhomboidaria (Willow Beauty: rural to industrial); Phigalia pilosaria (Pale Brindled Beauty: rural to industrial). Lymantriidae: Lymantria dispar (Gypsy Moth: sex-linked rural melanism predominant in males); Lymantria monacha (Black Arches: rural). Noctuidae: Allophyes oxyacanthae (Green-Brindled Crescent: rural to industrial; Apamea monoglypha (Dark Arches: rural to industrial); Apamea crenata (Clouded-bordered Brindle: rural to industrial); Mesapamea secalis (Common Rustic: rural to indistrial); Noctua fimbriata (Broad-bordered Yellow Underwing: rural?); Noctua pronuba (Large Yellow Underwing: rural to industrial?); Oligia strigilis (Marbled Minor: rural to industrial?); Polia nebulosa (Grey Arches: fully industrial). Oecophoridae: Diurnea fagella (March Dagger: rural to industrial). Thyatiridae: Ochropacha duplaris (Common Lutestring: rural to industrial). Zygynidae: Adscita geryon (Cistus Forester: rural?).
The photographs of specimens from Watson’s collection in effect constitute a snapshot of melanism as manifested around the English North Staffordshire market town of Leek in the decade 1948–1958. No mercury vapour light-trap being then available, moths were collected by day mainly while searching tree trunks and disturbing foliage, and by night using a pressure lantern in the field, at street lamps or at lighted windows; supplemented by searching for larvae and by ‘pupa digging’. The localities given in the legends, though all ostensibly ‘rural’, are located about 10 miles and 30 miles respectively from the heavily industrialised and at that time chokingly smog-afflicted regions of Stoke-on-Trent and Manchester. If implementation from the early 1960s of the ‘Clean Air Act’ has had a noticeable effect on industrial melanism in the moth fauna around Leek (see Majerus, 2002), this should by now be readily detectable in the resurgence in these locations of the formerly ‘normal’ (i.e., light) forms of such common species as the Peppered Moth, the Pale Brindled Beauty and the Mottled Beauty.