Peppered moth evolution
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 which they rested upon. However, because of widespread pollution during the Industrial Revolution in England, many of the lichens died out, and the trees that peppered moths rested on became blackened by soot, causing most of the light-coloured moths, or typica, to die off from predation. At the same time, the dark-coloured, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.
Since then, with improved environmental standards, light-coloured 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.
Evolution is defined as "a change in the frequency of an allele within a gene pool", an occurrence that causes a population's genetically inherited traits to change over successive generations. Evolution in the wild is chiefly caused by two mechanisms: natural selection, the process by which individual organisms with beneficial traits are more likely to survive and reproduce, and genetic drift, the statistical drift over time of allele frequencies in a population from random sampling effects in the formation of successive generations.
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 those morphs that were not camouflaged with increased frequency.
In 1924, J.B.S. Haldane calculated, using a simple general selection model, the selective advantage necessary for the recorded natural evolution of peppered moths, based on the assumption that in 1848 the frequency of dark-coloured moths was 2%, and by 1895 it was 95%. The dark-coloured, or melanic, form would have had to be 50% more fit than the typical, light-coloured form. Even taking into consideration possible errors in the model, this reasonably excluded the stochastic process of genetic drift, because the changes were too fast. Haldane's statistical analysis of selection for the melanic variant in peppered moths became a well known part of his effort to demonstrate that mathematical models that combined natural selection with Mendelian genetics could explain evolution — an effort that played a key role in the foundation of the discipline of population genetics, and the beginnings of the modern evolutionary synthesis.
In peppered moths, the allele for dark-bodied moths is dominant, while the allele for light-bodied moths is recessive, meaning that the typica moths have a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele), and never in a heterozygous one. This helps explain how dramatically quickly the population changed when being selected for dark colouration.
The peppered moth Biston betularia is also a model of parallel evolution in the incidence of melanism in the British form (f. carbonaria) and the American form (f. swettaria) as they are indistinguishable in appearance. Genetic analysis indicates that both phenotypes are inherited as autosomal dominants. Cross hybridizations indicate the phenotypes are produced by isoalleles at a single locus.
Before the Industrial Revolution, the peppered moth was mostly found in a light grey form with little black speckled spots. The light-bodied moths were able to blend in with the light-coloured lichens and tree bark, and the less common black moth was more likely to be eaten by birds. As a result of the common light-coloured lichens and English trees, therefore, the light-coloured moths were much more effective at hiding from predators, and the frequency of the dark allele was about 0.01%.
During the early decades of the Industrial Revolution in England, the countryside between London and Manchester was blanketed with soot from the new coal-burning factories. Many of the light-bodied lichens died from sulphur dioxide emissions, and the trees became covered with soot. This led to an increase in bird predation for light-coloured moths, as they no longer blended in as well in their polluted ecosystem: indeed, their bodies now dramatically contrasted with the colour of the bark. Dark-coloured moths, on the other hand, were camouflaged very well by the blackened trees.
Although a majority of light-coloured moths initially continued to be produced, most of them didn't survive, while the dark-coloured moths flourished. As a result, over the course of many generations of moths, the allele frequency gradually shifted towards the dominant allele, as more and more dark-bodied moths survived to reproduce. By the mid-19th century, the number of dark-coloured moths had risen noticeably, and by 1895, the percentage of dark-coloured moths in the Manchester peppered moth population was reported at 98%, a dramatic change (by almost 100%) from the original frequency. This evolved darkening in colour as a result of industrialization has come to be known as industrial melanism as a result.
While evidence of increasing frequency of melanic forms in the Lepidoptera was available during Darwin’s lifetime – the first observations were made in 1848 – current understanding is that it was not until 1896, 14 years after Darwin’s death, that Tutt explicitly linked melanism with natural selection. However, a recent article reports that melanism in the Lepidoptera had been linked to natural selection prior to Tutt. Albert Brydges Farn (1841–1921), a British entomologist, wrote to Darwin on 18 November 1878 to discuss his observation of colour variations in the Annulet moth (then Gnophos obscurata, now Charissa obscurata). In his letter, Farn mentions the existence of different colour morphs, describing how each is matched to the habitats in which they are found (dark morphs on peat, white morphs on chalk cliffs) and refers explicitly to this variability as pointing to ‘survival of the fittest’.
In modern times, because of cleaner air standards in Europe and North America, the dark-bodied moth is becoming less frequent, again demonstrating the adaptive shifts in the peppered moth population.
Rise and fall of phenotype frequency
Melanism has appeared in the European and North American peppered moth populations. Information about the rise in frequency is scarce. Much more is known about the subsequent fall in phenotype frequency, as it has been measured by lepidopterists using moth traps.
Though a black peppered moth was found in 1811, this can be seen as an aberrant morph caused by a recurrent mutation that was probably selected out of the population. The first carbonaria to be found was caught in Manchester, England in 1848, but was only reported 16 years later in 1864 by Edleston. Edleston notes that by 1864 it was the more common morph in his garden in Manchester. Steward compiled data for the first recordings of the peppered moth by locality, and deduced that the carbonaria morph was the result of a single mutation that subsequently spread. By 1895, it had reached a reported frequency of 98% in Manchester.
From around 1962 to the present, the phenotype frequency of carbonaria has steadily fallen. Its decline has been measured more accurately than its rise, because of more rigorous scientific studies being conducted. Notably, Bernard Kettlewell conducted a national survey in 1956, Bruce Grant conducted a similar one in early 1996. , and L.M. Cook in 2003.
Similar results were found 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.
In 1896 J. W. Tutt hypothesised that the increased proportion of carbonaria resulted from differential bird predation giving an advantage to the melanistic phenotype in polluted regions, but not in unpolluted regions where the light coloured typica moths had the advantage. Various experiments have been performed on predation of the peppered moth and each has supported this hypothesis.
The most famous experiments on the peppered moth were carried out by Bernard Kettlewell under the supervision of E. B. Ford, who helped him gain a grant from the Nuffield Foundation to perform the experiments. In one of Kettlewell's experiments, moths were released into a large (18 m by 6 m) aviary, where they were fed on by great tits (Parus major). In 1953, Kettlewell experimented at Cadbury Nature Reserve in Birmingham, England, marking, releasing and recapturing marked moths. He found that in this polluted woodland typica morphs were preferentially preyed. He thus showed that the melanistic phenotype was important to the survival of peppered moths in such a habitat. Kettlewell repeated the experiment in 1955 at unpolluted woodland in Dorset and again in the polluted woods in Birmingham. He was accompanied by Nico Tinbergen, and they made a film together. Further studies by others found similar results, culminating in 1996 when reporting work on both sides of the Atlantic found a correlation between changes in melanic frequencies and pollution levels.
An experiment in field biology will always suffer from some level of artificiality, but that has to be balanced against practicality, costs and in this case the history of field biology; the most important aspect is that an experiment generates useful statistics. The only previous experiments of this type were R.A. Fisher and E.B. Ford's (1947) with the scarlet tiger moth.
Michael Majerus in his 1998 book Melanism: Evolution in Action discussed criticisms concerning Kettlewell's experimental methods. Controversy arose when the book was misrepresented in reviews, and the story was picked up by creationist campaigners. The journalist Judith Hooper suggested in her book Of Moths and Men (2002) that Kettlewell committed scientific fraud. Careful studies of Kettlewell's surviving papers by Rudge (2005) and Young (2004) have revealed that Hooper's allegation of fraud is unjustified, and "that Hooper does not provide one shred of evidence to support this serious allegation”.
In 2000 Majerus developed plans for experiments to establish where peppered moths rest through the day, and to examine if the various valid criticisms of Kettlewell’s experimental protocols could have altered the qualitative validity of his conclusions. In the following year he piloted a new field predation experiment designed to overcome criticisms that Kettlewell had used too few release sites, resulting in the density of moths being too high; moths had been released onto tree trunks rather than branches; moths released during the day might not have found the best places to hide; mixtures of wild-caught and lab-bred moths might have behaved differently; and the behaviour of translocated moths might have changed because of local adaptation. During the main experiment in Cambridge over the seven years 2001–2007 Majerus noted the natural resting positions of the 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. Following correspondence with Hooper he added an experiment to find if bat predation might have skewed the results – this found that bats preyed equally on both forms of the moth. He observed a number of species of bird preying on the moths, and the overall data led him to conclude that differential bird predation was a major factor responsible for the decline in carbonaria frequency compared to typica in Cambridge during the study period. He described his results as a complete vindication of the peppered moth story, and said "If the rise and fall of the peppered moth is one of the most visually impacting and easily understood examples of Darwinian evolution in action, it should be taught. It provides after all the proof of evolution."
Majerus died before he could complete the writing up of his experiments, but the work was carried on by Cook, Grant, Saccheri and Mallet, and published on 8 February 2012 as "Selective bird predation on the peppered moth: the last experiment of Michael Majerus." The experiment was the largest ever in the study of industrial melanism involving 4864 individuals in a six-year investigation, and they concluded that melanism in moths is a genuine example of natural selection upon camouflage and predation.
Several alternative hypotheses to explain industrial melanism, particularly noted in the peppered moth, were proposed during the 1920s and 1930s. Some dissenters within the scientific community have criticized the peppered moth story, notably Sargent et al. (1998), but peppered moth researchers remain unconvinced.
Several alternative selection mechanisms have been proposed. Note that a change in allele frequency, be it caused by natural selection, mutation, migration or genetic drift by definition, is differential. However, the magnitude of the changes observed can only be accounted for by natural selection. It can be seen from population genetics that a non-differential change will not cause evolution. If the allele frequencies are denoted by the algebraic terms p and q, and (say) p = 0.6 and q = 0.4, then a non-differential reduction in population size from say 2000 to 100 individuals, will still produce the same values of (approximately) p = 0.6 and q = 0.4.
A more recent hypothesis by Riley proposes an additional selective factor, where heavy metal chelation by melanin may protect peppered moths against the toxic effects of heavy metals associated with industrialisation. This selective advantage would supplement the major selective mechanism of differential bird predation.
John William Heslop-Harrison (1920) rejected Tutt's differential bird predation hypothesis, on the basis that he did not believe that birds ate moths. Instead he advocated the idea that pollutants could cause changes to the soma and germ plasm of the organism. The origin of this hypothesis probably has its roots in the 1890s, when it was proposed as a form of Lamarckism. It is important to note its historical context.
Hasebroek (1925) was the first to try to prove this hypothesis. He contended that air pollution altered lepidopteran physiology, thus producing an excess of black pigment. He exposed pupae of Lepidoptera to various doses of pollutant gases, namely hydrogen sulfide (H2S), ammonia (NH3) and "pyredin" (presumably his spelling of pyridine). He used eight species in his studies, four of which were species of butterfly that did not exhibit melanism. Ford (1964) contends that Hasebroek's illustrations showed that the abnormal forms that appeared were not melanics, and Hasebroek failed to study their genetics.
Heslop Harrison (Harrison and Garrett 1926; Harrison 1928) suggested that the increase of melanic moths in industrialised regions was due to "mutation pressure", not to selection by predators which he regarded as negligible. Salts of lead and manganese were present in the airborne pollutant particles, and he suggested that these caused the mutation of genes for melanin production but of no others. He used Selenia bilunaria and Tephrosia bistortata as material. The larvae were fed with leaves that had incorporated these salts and melanics subsequently appeared.
Similar experiments by Hughes McKenney (1932) and Thomasen and Lemche (1933) failed to replicate these results. However, the statistician and geneticist Ronald Fisher showed that Heslop Harrison's controls were inadequate. This hypothesis, however, appeared to be falsified by breeding experiments. Research into the biochemistry of melanism continues.
Melanism: Evolution in Action
Michael Majerus in his 1998 book Melanism: Evolution in Action questioned the original methodology of Bernard Kettlewell's classic experiments, matching a similar 1998 analysis by Sargent et al.. When the biologist Jerry Coyne reviewed this book in Nature, he stated that the most serious problem found by Majerus was that only two peppered moths had been found on tree trunks. He also wrote that the white moths had increased in numbers before the lichen had returned and that Kettlewell's findings of moths choosing matching backgrounds had not been replicated in later experiments. Coyne compared his reaction to "the dismay attending my discovery, at the age of 6, that it was my father and not Santa who brought the presents on Christmas Eve". He concluded that "for the time being we must discard Biston as a well-understood example of natural selection in action, although it is clearly a case of evolution. There are many studies more appropriate for use in the classroom" and that further studies of the animal's habits were needed.
In contrast to this review, Majerus had stressed that the wealth of additional data obtained since Kettlewell's initial predation papers had not undermined the basic findings from that work, and that differential bird predation of the dark and light moths in habitats affected by industrial pollution to different degrees (directional selection) "is the primary influence of the evolution of melanism in the peppered moth". Coyne had erred in his statement that only two peppered moths had been found on tree trunks, as the book gives the resting positions of 47 peppered moths Majerus had found in the wild between 1964 and 1996; twelve were on tree trunks (six exposed, six unexposed), twenty were at the trunk/branch joint, and fifteen resting on branches. Majerus found that the review did not reflect the factual content of the book or his own views, and cited an assessment by the entomologist Donald Frack that there was essentially no resemblance between the book and Coyne's review, which appeared to be a summary of the Sargent et al. paper rather than Majerus's book.
Majerus had died before writing up his study, but his work was completed by Cook et al. and published in February 2012, concluding that "These data provide the most direct evidence yet to implicate camouflage and bird predation as the overriding explanation for the rise and fall of melanism in moths." Coyne wrote an article summarising the unfolding of the controversy which had followed his original review, and said "Despite the defensiveness of British evolutionists, I think my criticisms carried some weight, because Cambridge biologist Michael Majerus decided to repeat Kettlewell’s experiments, but doing them correctly this time." He quoted the Cook et al. study's conclusion that "These new data answer criticisms of earlier work and validate the methodology employed in many previous predation experiments that used tree trunks as resting sites. The new data, coupled with the weight of previously existing data convincingly show that ‘industrial melanism in the peppered moth is still one of the clearest and most easily understood examples of Darwinian evolution in action’." Coyne said he was "delighted to agree with this conclusion, which answers my previous criticisms about the Biston story."
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Biologists agree that the peppered moth example shows natural selection causing rapid adaptation to a changing environment. Creationists argued that the "peppered moth story" showed only microevolution, rather than speciation or other changes at the larger macroevolutionary scale. Although creationists accept microevolution of varieties within a kind, they claim that macroevolution does not happen. To biologists there is no dividing line between the two, and in the modern evolutionary synthesis the same mechanisms are seen operating at various scales to cause both evolution within species and speciation at a macroevolution level or wider changes, the only difference being of time and scale.
The story of Coyne's review was taken up by intelligent design creationists, and at a seminar presenting the wedge strategy on 13 March 1999, creationist and professor of law Phillip E. Johnson said that the moths "do not sit on tree trunks", "moths had to be glued to the trunks" for pictures and that the experiments were "fraudulent" and a "scam." This led Frack to exchanges with intelligent design proponent Jonathan Wells, who conceded that Majerus listed six moths on exposed tree trunks (out of 47), but argued that this was "an insignificant proportion". Wells wrote an essay on the subject, a shortened version of which appeared in The Scientist of 24 May 1999, claiming that "In 25 years of fieldwork, C.A. Clarke and his colleagues found only one peppered moth on a tree trunk", and concluding that "The fact that peppered moths do not normally rest on tree trunks invalidates Kettlewell's experiments".
In 2000 Wells wrote Icons of Evolution: Why much of what we Teach About Evolution is Wrong, which claims "What the textbooks don't explain, however, is that biologists have known since the 1980s that the classical story has some serious flaws. The most serious is that peppered moths in the wild don't even rest on tree trunks. The textbook photographs, it turns out, have been staged." The arguments put by Wells have been dismissed by Majerus, Cook and peppered moth researcher Bruce Grant who describes Wells as distorting the picture by selectively omitting or scrambling references in a way that is dishonest. Professional photography to illustrate textbooks uses dead insects because of the considerable difficulty in getting good images of both forms of moth in the same shot. The scientific studies actually consisted of observational data rather than using such photographs. The photographs in Michael Majerus's 1998 book Melanism: Evolution in Action are unstaged pictures of live moths in the wild, and the photographs of moths on tree-trunks, apart from some slight blurring, look little different from the "staged" photographs. While an experiment did involve the gluing of dead moths to trees, this practice was just one of many different ways used to study different individual elements of the overall hypothesis. This particular experiment was not meant to exactly reproduce natural conditions but instead was used to assess how the numbers of moths available (their density) affected the foraging practices of birds.
On 27 November 2000, the school board of Pratt County, Kansas continued efforts to favor intelligent design teaching by requiring the use of specific resources. These included the article by Jerry Coyne, who wrote to object strongly to this creationist misrepresentation of his critical re-evaluation, emphasising that the moth story is a sound example of evolution produced by natural selection and stating that his call for additional research was only to resolve uncertainty regarding bird predation as the cause of the natural selection and evolutionary change. Bruce Grant also wrote to challenge allegations of fraud in the moth experiments based on misrepresentations by Wells.
In 2002, journalist Judith Hooper's Of Moths and Men made accusations of scientific fraud. She accused Kettlewell of manipulating his data to prove his hypothesis. The book received strong criticism from the scientific press (e.g., Coyne, B.C. Clarke, Grant). Majerus described it as "littered with errors, misrepresentations, misinterpretations and falsehoods".
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- 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.
- Young, Matt. "Moonshine: Why the Peppered Moth remains an Icon of Evolution". Department of Physics, Colorado School of Mines. Retrieved 2009-01-03.
- The Peppered Moth: Decline of a Darwinian Disciple. This is the transcript of Michael Majerus' lecture delivered to the British Humanist Association on Darwin Day 2004.
- The Peppered Moth: The Proof of Darwinian Evolution. This is the transcript of Majerus' lecture given at the European Society for Evolutionary Biology meeting on 23 August 2007. The accompanying powerpoint presentation is also available.
- Majerus, Michael E. N. (March 2009). "Industrial Melanism in the Peppered Moth, Biston betularia: An Excellent Teaching Example of Darwinian Evolution in Action". Evolution: Education and Outreach 2 (1): 63–74. doi:10.1007/s12052-008-0107-y. "Accusations of data fudging and scientific fraud in the case are found to be vacuous."
- On 19 June 2009, Telegraph.co.uk published an article on this evolutionary phenomenon and implored UK readers to visit the Moths Count website and record their observations of local moths, in an effort to help increase the available data for researchers.