Peppered Moth - Morphs

Morphs

There are several melanic and non-melanic morphs of the peppered moth. These are controlled genetically. A particular morph can be indicated in a standard way by following the species name in the form "morpha morph name".

It is a common mistake to confuse the name of the morph with that of the species or subspecies, hence mistakes such as "Biston carbonaria" and "Biston betularia carbonaria". This might lead to the erroneous belief that speciation was involved in the observed evolution of the peppered moth. This is not the case; individuals of each morph interbreed and produce fertile offspring with individuals of all other morphs; hence there is only one peppered moth species.

By contrast, different subspecies of the same species can theoretically interbreed with one another and will produce fully fertile and healthy offspring but in practice do not, as they live in different regions or reproduce in different seasons. Full-fledged species are either unable to produce fertile and healthy offspring, or do not recognize each other's courtship signals, or both.

In continental Europe, there are three morphs: morpha typica, the typical white morph (also known as "morpha betularia"), morpha carbonaria, the melanic black morph (also previously known as "morpha doubledayaria"), and morpha medionigra, an intermediate semi-melanic morph. European breeding experiments have shown that in Biston betularia betularia, the allele for melanism producing morpha carbonaria is controlled by a single locus. The melanic allele is dominant to the non-melanic allele. This situation is, however, somewhat complicated by the presence of three other alleles that produce indistinguishable morphs of morpha medionigra. These are of intermediate dominance, but this is not complete (Majerus, 1998).

In Britain, the typical white speckled morph is known as morpha typica, the melanic morph is morpha carbonaria, and the intermediate phenotype is morpha insularia.

In North America, the melanic black morph is morpha swettaria. In Biston betularia cognataria, the melanic allele (producing morpha swettaria) is similarly dominant to the non-melanic allele. There are also some intermediate morphs. In Japan, no melanic morphs have been recorded; they are all morpha typica.

At present, the precise molecular genetics and biochemistry of the melanism in this species remains unknown. True (2003) has reviewed this and suggests work based on candidate genes from other insects such as the fruit fly Drosophila melanogaster. In any case, it is rather likely that the underlying mechanism is not overly complex and, as indicated above, does not involve very many genes and alleles: Unlike for example the variation seen in human skin color, Peppered Moth morphs are not clinal and can generally be readily distinguished from another.