Peppered moth

Peppered moth
Biston betularia betularia morpha typica
Conservation status
Not evaluated (IUCN 3.1)
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Insecta Order: Lepidoptera Family: Geometridae Genus: Biston Species: B. betularia
Binomial name
Biston betularia Linnaeus, 1758
Subspecies
B. b. betularia B. b. cognataria B. b. parva

The peppered moth (Biston betularia) is a temperate species of night-flying moth. Peppered moth evolution is often used by educators as an example of natural selection.

Ecology and life cycle

Biston betularia caterpillars on birch (left) and willow (right), demonstrating twig mimicry. ^[1]

In Britain, the peppered moth is univoltine (i.e. it has one generation per year), whilst in south-eastern North America it is bivoltine (two generations per year). The lepidopteran life cycle consists of four stages: ova (eggs), several larval instars (caterpillars), pupae, which overwinter live in the soil, and imagines (adults). During the day, the moths typically rest on trees, where they are preyed on by birds.

The caterpillar is a twig mimic, varying in colour between green and brown. It goes into the soil late in the season, where it pupates in order to spend the winter. The imagines emerge from the pupae between late May and August, the males slightly before the females (this is common and expected from sexual selection). They emerge late in the day and dry their wings before flying that night.

The males fly every night of their lives in search of females, whereas the females release pheromones to attract males, only flying on the first night. Since the pheromone is carried by the wind, males tend to travel up the concentration gradient, i.e., towards the source. During flight, they are subject to predation by bats. The males guard the female from other males until she lays the eggs. The female lays about 2,000 pale-green ovoid eggs about 1 mm in length into crevices in bark with her ovipositor.

Resting behaviour

Figure 2a. Total number of observed moths = 59: Exposed trunk = 7; unexposed trunk = 7; trunk-branch joint = 23; branches = 22

Figure 2b. Total number of observed moths = 23: Exposed trunk =1; unexposed trunk =1; trunk branch join = 3; branches thicker than 5 centimetres diameter = 10; branches and twigs less thick than 5 centimetres = 8

A mating pair or a lone individual will spend the day hiding from predators, particularly birds. In the case of the former, the male stays with the female to ensure paternity. The best evidence for resting positions is given by data collected by the peppered moth researcher Michael Majerus, and it is given in the accompanying charts. These data were originally published in Howlett and Majerus (1987), and an updated version published in Majerus (1998), who concluded that the moths rest in the upper part of the trees. Majerus notes:

Creationist critics of the peppered moth have often pointed to a statement made by Clarke et al. (1985): "... In 25 years we have only found two betularia on the tree trunks or walls adjacent to our traps, and none elsewhere". The reason now seems obvious. Few people spend their time looking for moths up in the trees. That is where peppered moths rest by day.

From their original data, Howlett and Majerus (1987) concluded that peppered moths generally rest in unexposed positions, using three main types of site. Firstly, a few inches below a branch-trunk joint on a tree trunk where the moth is in shadow; secondly, on the underside of branches and thirdly on foliate twigs. The above data would appear to support this.

Further support for these resting positions are given from experiments watching captive moths taking up resting positions in both males (Mikkola, 1979; 1984) and females (Liebert and Brakefield, 1987).

Majerus et al (2000) have shown that peppered moths are cryptically camouflaged against their backgrounds when they rest in the boughs of trees. It is clear that in human visible wavelengths, typica are camouflaged against lichens and carbonaria against plain bark. However, birds are capable of seeing ultraviolet light that humans cannot see. Using an ultraviolet-sensitive video camera, Majerus et al showed that typica reflect ultraviolet light in a speckled fashion and are camouflaged against crustose lichens common on branches, both in ultraviolet and human-visible wavelengths. However, typica are not as well camouflaged against foliose lichens common on tree trunks; though they are camouflaged in human wavelengths, in ultraviolet wavelengths, foliose lichens do not reflect ultraviolet light.

During an experiment in Cambridge over the seven years 2001-2007 Majerus noted the natural resting positions of peppered moths, and of the 135 moths examined over half were on tree branches, mostly on the lower half of the branch, 37% were on tree trunks, mostly on the north side, and only 12.6% were resting on or under twigs.^[2][3]

Morphs

Further information: Polymorphism (biology)

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.

Evolution

Main article: Peppered moth evolution

Biston betularia betularia morpha typica, the white-bodied peppered moth.

Biston betularia betularia morpha carbonaria, the black-bodied peppered moth.

The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light coloration, which effectively camouflaged them against the light-colored trees and lichens which they rested upon. However, due to widespread pollution during the Industrial Revolution in England, many of the lichens died out, and the trees which peppered moths rested on became blackened by soot, causing most of the light-colored moths, or typica, to die off due to predation. At the same time, the dark-colored, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.^[4]

Since then, with improved environmental standards, light-colored peppered moths have again become common, but the dramatic change in the peppered moth's population has remained a subject of much interest and study, and has led to the coining of the term "industrial melanism" to refer to the genetic darkening of species in response to pollutants. As a result of the relatively simple and easy-to-understand circumstances of the adaptation, the peppered moth has become a common example used in explaining or demonstrating natural selection to laypeople and classroom students.^[5]

The first carbonaria morph was recorded by Edleston in Manchester in 1848, and over the subsequent years it increased in frequency. Predation experiments, particularly by Bernard Kettlewell, established that the agent of selection was birds who preyed on the morpha carbonaria morph.

Jonathan Wells is one of a number of creationists who have criticized the use of peppered moth melanism as an example of evolution in action. In his book Icons of Evolution, Wells alleges that peppered moth studies, and in particular Kettlewell's experiments, were erroneous and fraudulent.^[6] Similarly, in 2002 Judith Hooper repeatedly implied fraud and error in Kettlewell's experiments in her book titled Of moths and men.^[7] Despite some valid criticisms of the early experiments, there has been no evidence of fraud and subsequent experiments and observations have clearly shown the phenomenon and supported its initial explanation.^[6][8][9] The problem, according to the Young Earth creationist Dr. Tommy Mitchell of "Answers in Genesis", is that it only shows a case of natural selection, and not of evolution, as a population of a kind of moth turned into simply a population of another kind of moth.^[10] While it is true that this example shows natural selection causing microevolution within a species, it demonstrates rapid and obvious adaptiveness with such change,^[11] and despite the claims of creationists, there are no barriers preventing such changes from accumulating to form new species.^[12]

References

External links

• More on the peppered moth as an example of evolution
• Bruce Grant has written several papers on melanism in the peppered moth which are listed on his home page.
• Online lecture: "The rise and fall of the melanic Peppered Moth" presented by Laurence Cook.
• Matt Young. "Talk Reason: arguments against creationism, intelligent design, and religious apologetics". Moonshine: Why the Peppered Moth remains an Icon of Evolution. Department of Physics, Colorado School of Mines. Retrieved on 2007-09-10.
• Michael Majerus (2004). "The Peppered moth: decline of a Darwinian disciple" (.doc). Retrieved on 2007-09-10.