The peppered moth (Biston betularia (L.)) is a temperate species of night-flying moth notable for industrial melanism in Europe and North America, but particularly in Great Britain. It is often cited as an example of Charles Darwin's theory of evolution by natural selection in action. However, the experimental work is constructively criticised by many scientists. In addition, the peppered moth story is strongly criticised by a few scientists and and is widely attacked by creationists.
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| Biston betularia Linnaeus, 1758 |
Nomenclature
Common Names
In the English language, Biston betularia is known as the "peppered moth" because of speckles on the wings of the typica morph that make it look like it is dusted with pepper. An older name is the "salt and pepper moth". In the German language it is known as "Birkenspanner", in the Dutch language "Berkespanner" and in the French language "Phalène du bouleau" - all meaning "birch moth" (the birch tree being one of its principle food sources). In the Japanese language it is known as "Oo-shimofuri-eda-shaku" meaning "frosted branch-measuring moth".
Taxonomy
For a full scientific classification see the table on the right. There are has three subspecies, one in each of the continents that is occupied:
- Biston betularia betularia is the European subspecies.
- Biston betularia cognataria is the North American subspecies.
- Biston betularia parva is the Japanese subspecies.
These were previously identified as separate species, so in older texts are occasionally written as Biston cognataria, etc.
There is also one synonymous name [note to self; put in. see Kettlewell 1973]
Melanism
Descriptions and nomenclature
There are different morphs of this moth which has melanic and non-melanic forms. This has importance to the case study of evolution mentioned below. A particular morph can be indicated in a standard way by following the species name in the form "f. morph".
NB: 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 confusion 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:
- f. typica; the typical white morph (also known as f. betularia)
- f. carbonaria the melanic black morph, (also previously known as f. doubledayaria)
- f. medionigra, an intermediate semi-melanic morph.
In North America, the typical white morph is also known as f. typica; the melanic black morph is f. swettaria.
In Japan, no melanic morphs have been recorded; they are all f. typica.
Genetics
Breeding experiments have shown that in Biston betularia betularia, the allele for melanism producing f. 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 f. medionigra. These are of intermediate dominance, but this is not complete. See Majerus (1998).
In Biston betularia cognataria, the melanic allele (producing f. swettaria) is similarly dominant to the non-melanic allele. There are no intermediate morphs.
Biochemistry
- See biochemistry
Unfortunately, the precise pathways that cause melanism are at present unknown. However, True (2003) gives a summary of this.
Ecology
Larvae
- see larva
First instar larvae hang themselves from a silk thread and as such can disperse as part of the aerial plankton.
Pupae
- see pupa
Adults
- see imago
Peppered moths inhabit woodland canopies. They are night-flying and the flight period is from May to August. During flight they are subject to predation by bats.
Males fly every night, and may travel large distances. They will be attracted by pheromones released by females, and so will tend to fly upwind.
Females only fly one night after eclosion which tends to be short (Brakefield and Liebert, 1990).
During the day peppered moths rest. Moths in general are predated by birds during the day. It is believed that the peppered moth is no exception, and birds have been observed predating them (see predation experiments below).
Evidence for the precise resting ___location is provided by a data set collected by Majerus (2004) slide 42, an earlier version of which is interpreted in Howlett and Majerus (1987). From these data, Howlett and Majerus 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 treetrunk where the moth is in shadow; secondly, on the underside of branches and thirdly on foliate twigs.
It is however worth noting the problems with this is sample. Like any sample rather than a census, it is a biased sample. This however has the added problem of its small size. As moths on trunks are easier to spot, this sample might tend to be biased against moths resting on the trunks of trees. On the other hand, moths resting on tree trunks would probably be more susceptible to predation by birds, and if they were eaten before the lepidopterist walked by, they would not be recorded. See ecological sampling techniques.
This is further supported by experiments watching captive moths taking up resting positions in both males (Mikkola, 1979; 1984) and females (Liebert and Brakefield, 1987).
Evolution
Evolution, more specifically microevolution is defined as "a change in the frequency of an allele within a gene pool" (Dobzhansky, 1937); see population genetics. Since the genetics of how the morphology is controlled are known (see above), changes in phenotype frequency changes indicate such allele frequency changes.
Rise and fall of phenotype frequency
Melanism has appeared in the European and North American populations. This considers Britain and Europe first, then North America, then Japan.
The first carbonaria was found in Manchester, England in 1848, but was only reported 16 years later in 1864 by Edleston. Edleston notes that by 1864 it was the commoner morph in his garden in Manchester. Steward (1977) compiled data for the first recordings of the peppered moth by locality, and deduced that carbonaria morph was the result of a single mutation that subsequently spread. In Manchester by 1895 it had reached a reported frequency of 98%.
From c.1962 to present (2004), the phenotype frequency of carbonaria has fallen. Its decline has been measured more accurately than its rise; see moth trapping. See Grant et al (1995).
Similar results in America.
Melanic forms have not been found in Japan. It is believed that this is because peppered moths in Japan do not inhabit industrialised regions.
Mechanisms of evolution
Evolution in the wild is caused by two mechanisms; natural selection; and genetic drift.
J.B.S. Haldane in 1924 calculated the selective advantage necessary for the evolution that had been recorded, based on the assumption that in 1848 the frequency was 2% and by 1895 95%. The melanic form would have to be one and a half times as fit as the typical form. This reasonably excluded the stochastic process of genetic drift because the changes were too fast.
J.W. Tutt first proposed the "differential bird predation hypothesis" in 1896, as a mechanism of natural selection. The melanic morphs were better camouflaged against the bark of trees without foliose lichen, whereas the typica morphs were better camouflaged against trees with lichens. As a result, birds would find and eat with increased frequency those morphs that weren't camouflaged.
Experimental Work
Predation Experiments
Bernard Kettlewell conducted predation experiments, including direct observation of bird predation on moths (under controlled conditions and in the wild), and mark-release-recapture experiments, where moths are released into a forest and then recaptured. Kettlewell performed reciprocal experiments in polluted and unpolluted woods, and found that typica moths were more likely to survive predation in unpolluted woods, and carbonaria moths in polluted woods (Kettlewell, 1955a; 1956; see also Kettlewell (1973)).
These experiments have been criticised for their artificiality. Kettlewell placed dead morphs on the trunks, not the branches of the tree, at too high density He marked them for conspicuousness himself, thus violating the double-blind principle. In addition, it has subsequently been found that bird eye detect Ultra-Violet radiation whereas human eyes do not.
Resting background choice experiments
- Kettlwell's Experiments
- Sargent's Experiments
- Howlett and Majerus theory of co-evolution.
Attacking the Peppered Moth Paradigm
Evolution in general is attacked by creationists.
Alternative Selection Mechanisms
The phenotype changes can only be accounted for by natural selection. What are the alternatives then
Selection for resistance to pollution
Selection for resistance to pollution. Possible, but unlikely. Advocating this would however, appear to violate Occam's razor.
Differential squirrel predation
Selection by predation by squirrels? Possible, but seems mostly birdies.
Differential bat predation
It has been suggested by Hooper (2002) that the agent of selection was not birds on adult moths but bats on adult moths. From population genetics, it can be shown that only differential selection could cause the changes observed. A non-selective reduction in population size would keep the allele frequencies as they were before the reduction. This "differential bat predation" hypothesis has been tested by Mike Majerus (2004), who found that predation by pipestrel bats was non-selective.
[NB: I have these data. need to do significance tests on them]
Differential predation on larvae
A similar suggestion was made by Hooper regarding predation on larvae (and a similar suggestion could be made regarding predation on the pupae).
The Phenotypic Induction Theory
The phenotypic induction theory was first proposed by John William Heslop Harrison in 1920. He rejected Tutt's differential bird predation hypothesis, instead advocating the idea.
It is important to consider the historical context of such work. Ronald Fisher's 1918 model showing that discontinuous variation was the result of variation at many loci is traditionally seen as the start of the modern synthesis, which developed through the 1920s. Though this difficult paper was often ignored by non-mathematical biologists at the time [find a suitable quotation]. Lamarckism
This theory however appeared to be falsified by breeding experiments. Even if this model were true, differential bird predation would still be a selection.
Allegations of Fraud
It has been suggested by the journalist Judith Hooper in her book Of Moths and Men (Hooper, 2002) that Kettlewell committed scientific fraud. Her justification is (1) Kettlewell's mark-release-recapture rates rose, (2) that weather patterns in Birmingham at the time based on records from Birmingham Airport did not change (and therefore, by her reasoning, could not account for the change in recapture rates); and (3) a letter to Kettlewell from E.B. Ford dated 1st July 1953 included the phrase "It is disappointing that the recoveries are not better... ...however, I do not doubt that the results will be very worthwhile...", which Hooper implies made Kettlewell fabricate his results. Also, she appears to have a mistrust of lepidopterists. See also conspiracy theory.
However, the physicist Matt Young (2004) has shown that the changes in recapture rates are not statistically significant, and has also shown that the changes inversely correlate to levels of moonlight. Further, the dates of the letter and the increase in recapture rates do not match well, with the increase recapture rates preceeding the arrival of the letter. Also because of the unnaturally high release rates in Kettlewell's experiment, local birds may have become satiated and thus predation rates would have fallen. In addition, later experiments would appear to support Kettlewell's conclusions.
Creationism
Traditionally, claims have been limited to pointing out that the "peppered moth story" showed only microevolution rather than macroevolutionary trend of speciation (e.g. Watchtower Bible and Tract Society, 1985) but this is not disputed by scientists. As previously noted, the peppered moth - at present - has only one species.
From 1998 onwards however, this changed. A review by evolutionary biologist Jerry Coyne of Majerus's 1998 book Melanism: Evolution in Action was published in Nature. It famously included the following:
- [He compared his reaction to] "the dismay attending my discovery, at age 6, that it was my father and not Santa who brought the presents on Christmas Eve."
- "For the time being we must discard Biston as a well-understood example of natural selection in action…
However, it appears that Coyne was actually . Secondly, he claims he is misquoted.
This was subsequently picked up by Sunday Telegraph journalist Robert Matthews, who wrote:
- "Evolution experts are quietly admitting that one of their most cherished examples of Darwin’s theory, the rise and fall of the peppered moth, is based on a series of scientific blunders. Experiments using the moth in the Fifties and long believed to prove the truth of natural selection are now thought to be worthless, having been designed to come up with the “right” answer."
Subsequently, the creationist theologian and biochemist Jonathan Wells wrote reviews (2000; 2001; 2002). One of which appeared in The Scientist A search using a search engine will produce a wealth of similar results.
Then, Hooper's Of Moths and Men Came out, which claimed that Bernard Kettlewell had committed scientific fraud.
There are too many online reviews to list here. Notably though, common terms include "piltdown moth" (a reference to piltdown man), and slightly humourously "peppered myth".
The social importance of Biston betularia
The peppered moth has become part of "scientific mythology". Why?
Along with his conclusions, Kettlewell published striking photographs of each type of moth against the trunk of different trees. These photographs are often reproduced in biology textbooks. There is also Tinbergen's film.
Recommended Further Reading
Detailed Reviews
For a detailed review see Majerus (1998)
Short reviews
For a short online review see Grant (1999) in Heredity (available http://faculty.wm.edu/bsgran/melanism.pdf).
References
- Brakefield P.M. and Liebert T.G. (1990). The reliability of estimates of migration in the peppered moth Biston betularia and some implications for selection-migration Biological Journal of the Linnean Society 39 327-334.
- Coyne, J.A. (1998). Not black and white. Review of Melanism: Evolution in Action by Michael E.N. Majerus. Nature 396:35-36.
- Dobzhansky T.G. (1937). Genetics and the Evolutionary Process
- Edleston R.S. (1864) Untitled (first f. carbonaria melanic morph Biston betularia). Entomologist 2:150
- Fisher, R.A. (1918). The correlation between relatives on the supposition of Mendelian inheritance Transaction of the Royal Society of Edinburgh 52:399-433.
- Grant B.S. (1999). Fine tuning the peppered moth paradigm. Evolution 53:980-984
http://faculty.wm.edu/bsgran/melanism.pdf
- Grant B.S. Owen D.F. and Clarke C.A. (1995). Decline of melanic moths. Nature 373:565
- Haldane J.B.S. (1924). A mathematical theory of natural and artificial selection. Transactions of the Cambridge Philosopical Society 23:19-41
- Harrison, J.W.H. (1920).
- Harrison, J.W.H. (1926a). The inheritance of wing colour and pattern in the Lepidopterous genus Tephrosia (Ectropis).II. Experiments involving melanic Tephrosia bistortata and typical T. crepuscularia. Journal of Genetics 17:1-9
- Harrison, J.W.H. (1926b). Miscellaneous observations on the induction,incidence and iheritance of melanism in the Lepidoptera. Entomologist 59:121-123
- Harrison, J.W.H. (1927a). The inheritance of melanism between continental Tephrosia crepuscularia and British T. bistortata, with some remarks on the origin ofparthenogensis in interspecific crosses. Genetics 9:467-480
- Hooper J. (2002). Of Moths and Men.
- Howlett R.J. and Majerus, M.E.N. (1987) The understanding of industrial melanism in the peppered moth (Biston betularia) (Lepidoptera: Geometridae). Biological Journal of the Linnaean Society 30:31-44.
- Kettlewell H.B.D. (1955a). Selection experiments on industrial melanism in the Lepidoptera. Heredity 9:323-242.
- Kettlewell H.B.D. (1956). Further selection experiments on industrial melanism in the Lepidoptera. Heredity 10:287-301.
- Kettlewell H.B.D. (1973). Melanism.
- 'Kirby W.F. (1882). European Butterflies and Moths Cassel, Petter,
Galpin & Co: London.
- Liebert and Brakefield P.M. (1987). Behavioural studies in the peppered moth Biston betularia and a discussion on the role of pollution and epiphytes in industrial melanism. Biological Journal of the Linnean Society 31 129-150.
- Majerus M.E.N. (1998) Melanism - Evolution in Action. Oxford University Press, New York.
- Majerus M.E.N. (2004) The Peppered moth: decline of a Darwinian disciple. British Humanist Association Darwin Day Lecture. http://www.gen.cam.ac.uk/newdept/research/labs/Majerus/
- Mikkola K. (1979). Resting site selection of Oliga and Biston moths (Lepidoptera: Noctuidae and Gemoteridae). Acta Entomologica Fenici 45 81-87.
- Mikkola K. (1984). On the selective force acting in the industrial melanism of Biston and Oliga moths(Lepidoptera: Noctuidae and Gemoteridae). Biological Journal of the Linnean Society 21 409-421.
- Porritt G.T. (1926). The induction of melanism in the Lepidoptera and its subsequent inheritnce. Entomologist's Monthly Magazine 62:107-111
- Steward R.C. (1977). Industrial and non-industrial melanism in the peppered moth Biston betularia (L.) Ecological Entomology 2 231-243.
- True, J.R. (2003). Insect melanism: the molecules matter. Trends in Ecology and Evolution 18:640-647
- Tutt J.W.' (1896). British Moths. George Routledge: London.
- Watchtower Bible and Tract Society, (1985). Life--How Did It Get Here? Brooklyn, NY. See http://www.talkorigins.org/indexcc/CB/CB910_2.html
- Wells J. (2000) Icons of Evolution: Science or Myth? Why Much of What We Teach About Evolution is Wrong. Regnery Press, Washington, D.C.
- Wells, J. (2001) Second thoughts about peppered moths: This classical story of evolution by natural selection needs revising. The True Origin Archive http://trueorigin.org/pepmoth1.htm
- Wells, J. (2002) Moth-eaten statistics: A reply to Kenneth R. Miller. Discovery Institute: Center for Renewal of Science and Culture – Article Database, http://www.discovery.org/viewDB/index.php3?command=view&id=1147&program=CRSC
- Young, M. (2003). Moonshine: Why the Peppered Moth Remains an Icon of Evolution. http://www.talkreason.org/articles/moonshine.cfm or http://www.talkreason.org/articles/moonshine.cfm
Appendices
Appendix 1. Significant Persons
The following are significant persons.