Francais | English | Espanõl

Peppered moth

From Wikipedia, the free encyclopedia

Jump to: navigation, search

iPeppered Moth

Image:Biston.betularia.7200.jpg

Conservation status

Secure

</div>

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 often used by educators as an example of natural selection.

1 Ecology and life cycle
2 Genetics
3 Evolution
- 3.1 The "textbook" account - evolution in action
- 3.2 Criticisms of the "textbook" account
4 Further reading
5 References
6 External links

[edit] Ecology and life cycle

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 larva instars (caterpillars), pupae and imagines (adults). During the day, the moths cryptically rest on trees, where they are preyed on by birds.

[edit] Pupae

The pupae overwinter in the soil.

[edit] Imagines

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 who release pheromones to attract males only fly on the first night. Since the pheromone is carried by the wind, males tend to travel up the concentration gradient, i.e. upwind. During flight they are subject to predation by bats.

[edit] Imaginal diurnal resting behaviour

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 Dr. Mike 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 continues:

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 the 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).

[edit] Cryptic resting behaviour

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 are can not. 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.

[edit] Mating behaviour

The female releases pheromones which attract the males. The males guard the female from other males until she lays the eggs. The female lays about 2000 pale-green ovoid eggs about 1 mm in length into crevices in bark with her ovipositor.

[edit] Larvae

The caterpillar (see fig 1c) is a twig mimic, varying in colour between green and brown. Late in the season it goes into the soil, where it pupates in order to spend the winter, and the life cycle continues.

[edit] Genetics

The are several melanic and non-melanic morphs. A particular morph can be indicated in a standard way by following the species name in the form "morpha morph name". 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. These are controlled genetically. At present the biochemistry of the melanism remains unknown, though it should be possible for it to be elucidated.

[edit] Morphs

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 there has been demonstrable speciation in the case study mentioned below. This is not the case; individuals of each morph can breed with individuals of all other morphs, and hence there is only one species.

In 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")
morpha medionigra, an intermediate semi-melanic morph.

In North America, the typical white morph is also known as morpha typica; the melanic black morph is morpha swettaria.

In Japan, no melanic morphs have been recorded; they are all morphaf. typica.

[edit] Europe

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 Biston betularia cognataria, the melanic allele (producing morpha swettaria) is similarly dominant to the non-melanic allele. There are also some intermediate morphs.

[edit] Biochemistry

The molecular genetics and biochemistry of melanism in this species are at present unknown. True (2003) reviews this and suggests work based on candidate genes from other insects such as the fruit fly, Drosophila melanogaster.

[edit] Industrial melanism

The first carbonaria morph was recorded by Edleston in Manchester in 1848, and over the subsequent years it increased in frequency. This evolution was attributed to natural selection (i.e., too fast to be due to genetic drift), though the increase was not monitored very effectively.

Predation experiments (see below) particularly by Bernard Kettlewell established that the agent of selection was birds who preyed on the morpha carbonaria morph.

The Clean Air Acts reduced levels of pollution, and typica morph frequency has increased, again generally seen as an example of natural selection.

[edit] Evolution

Main article: Peppered moth evolution

Image:Biston.betularia.7200.jpg

This section first presents the analysis which caused the peppered moth to be regarded as an example of evolution in action and then very briefly summarises more recent scientific criticisms of this view.

[edit] The "textbook" account - evolution in action

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.<ref name="miller">Miller, Ken (1999). The Peppered Moth: An Update</ref>

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.<ref name="globalchange">A modelling exercise for students using the peppered moth as its example</ref>

[edit] Criticisms of the "textbook" account

Kettlewell's study in the 1950s, which is the basis of the "textbook" account, was based on 3 experiments in which he released similar numbers of light- and dark-coloured moths in Dorset (rural) and Birmingham (industrial, heavily polluted at that time) and then estimated how many of each type were taken by predators (birds). The 3 experiments used different methods of estimating how many moths fell victim to predators.

Later studies which sampled the percentages of light- and dark-coloured peppered moths living in unpolluted rural areas and in polluted industrial areas showed several measurements that seem to argue against the conventional story, for example:

In rural Wales there was an unexpectedly high proportion of dark-coloured moths.
In polluted south Wales dark-coloured forms were only 20% of the total population of peppered moths.
In rural East Anglia, dark-coloured forms were 80% of the total population of peppered moths.
A study of 165 separate sites in Britain found that there was a correlation between melanism and the concentration of sulfur dioxide north of latitude 52°N, but the correlation south of that latitude was less clear.

After pollution control legislation was implemented in the UK:

The proportion of dark forms decreased north of London (as expected), but surprisingly increased to the south.
Kettlewell and others found that light-coloured forms became more common before lichens re-appeared on trees in previously polluted areas. This undermines the idea that the lichens were the sole factor in causing light colored moths to be more common.

Studies in the USA also found no correlation between the amount of lichen on trees and the ratio of light- and dark-coloured peppered moths.

There have also been criticisms of the procedure Kettlewell used in his experiments. In particular:

In normal circumstances, Peppered Moths rest much more often on the undersides of small, more or less horizontal branches than tree trunks.
Kettlewell released his moths in the morning to avoid distorting their behaviour by using artificial light. But releasing the moths in the morning confused the moths so that they landed on the first moderately suitable objects they saw, which were tree-trunks. The moths would therefore have been much more exposed to predation than usual, irrespective of their coloration.

[edit] Further reading

Melanism: Evolution in Action (1998) by Mike Majerus is the definitive reference on this work, which provides an introduction to evolutionary theory as well as describing the Peppered Moth case study.