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Peppered moth
The melanic carbonaria (left) and the more common light-colored typica (right).
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.[1] Peppered moth evolution is often used by educators as an example of natural selection.[2]

Contents

Ecology and life cycle

Biston betularia caterpillars on birch (left) and willow (right), demonstrating twig mimicry. [3]

In Britain and Ireland, 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 only fly on the first night. Thereafter, the females release pheromones to attract males. Since the pheromone is carried by the wind, males tend to travel up the concentration gradient, i.e., toward 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.

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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 is 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.[4][5]

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.

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 colouration, which effectively camouflaged them against the light-coloured trees and lichens upon which they rested. 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-coloured moths, or typica, to die off due to predation. At the same time, the dark-coloured, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.[6]

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. This 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.[7]

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.[8] Similarly, in 2002 Judith Hooper repeatedly implied fraud and error in Kettlewell's experiments in her book titled Of moths and men.[9] Despite some valid criticisms of the early experiments, there has been no evidence of fraud. Subsequent experiments and observations have supported the initial explanation of the phenomenon.[8][10][11] But the problem, according to the Young Earth creationist Dr. Tommy Mitchell of "Answers in Genesis", is this only represents 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.[12] While it is true that this example shows natural selection causing microevolution within a species, it demonstrates rapid and obvious adaptiveness with such change.[13]

References

  1. ^ Grant, Bruce S (1999). "Fine tuning the peppered moth paradigm" (PDF). Evolution. pp. 53 980–984. http://bsgran.people.wm.edu/melanism.pdf. Retrieved 2009-12-19.  
  2. ^ Alan Gishlick (November 23, 2006). "Icon 6 — Peppered Moths". National Center for Science Education. http://ncse.com/creationism/analysis/icon-6-peppered-moths. Retrieved 2009-12-19.  
  3. ^ Noor MA, Parnell RS, Grant BS (2008). "A Reversible Color Polyphenism in American Peppered Moth (Biston betularia cognataria) Caterpillars". PLoS ONE 3 (9): e3142. doi:10.1371/journal.pone.0003142. PMID 18769543. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0003142.  
  4. ^ Michael E. N. Majerus (August 2007). "The Peppered Moth: The Proof of Darwinian Evolution" (PDF). http://www.gen.cam.ac.uk/Research/Majerus/Swedentalk220807.pdf. Retrieved 2007-09-09.  
  5. ^ Steve Connor, Science Editor (25 August 2007). "Moth study backs classic 'test case' for Darwin's theory - Independent Online Edition > Sci_Tech". The Independent. http://news.independent.co.uk/sci_tech/article2893896.ece. Retrieved 2007-09-09.  
  6. ^ Ken Miller (August 1999). "The peppered moth: an update". Brown University. http://www.millerandlevine.com/km/evol/Moths/moths.html.  
  7. ^ "A modelling exercise for students using the peppered moth as its example". http://www.globalchange.umich.edu/globalchange1/current/labs/peppered_moth/pepperedmoth.html.  
  8. ^ a b Marc Isaak (2005-05-02). "The peppered moth story". TalkOrigins Archive. Index to Creationist Claims: CB601. http://www.talkorigins.org/indexcc/CB/CB601.html.  
  9. ^ Bryan Clarke (2003). "Heredity - The art of innuendo". Heredity. pp. 90, 279–280. http://www.nature.com/hdy/journal/v90/n4/full/6800229a.html. Retrieved 2008-07-11.  
  10. ^ Marc Isaak (2003-09-10). "Peppered moths affected by mutagens". Index to Creationist Claims: CB601.2.3. http://www.talkorigins.org/indexcc/CB/CB601_2_3.html.  
  11. ^ Marc Isaak (2003-09-10). "Dark peppered moths unexpectedly common in areas". Index to Creationist Claims: CB601.2.2. http://www.talkorigins.org/indexcc/CB/CB601_2_2.html.  
  12. ^ Dr. Tommy Mitchell (2005-05-02). "Much Ado About Moths". Answers in Genesis. http://www.answersingenesis.org/articles/am/v3/n2/much-ado-about-moths.  
  13. ^ "Icon of Obfuscation". TalkOrigins Archive. http://www.talkorigins.org/faqs/wells/iconob.html#moths. Retrieved 2008-05-22.  

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