Peppered moth evolution


 * This article is about the peppered moth's significance in evolutionary biology. For its evolutionary ancestry, see Insect 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 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.

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.

Genetics
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 due to random sampling effects in the formation of successive generations.

In 1924, J.B.S. Haldane calculated, using a simple general selection model, the selective advantage necessary for the recorded evolution of peppered moths, based on the assumption that in 1848 the frequency of dark-colored moths was 2%, and by 1895 it was 95%. The dark-colored, or melanic, form would have had to be one and a half times as fit as the typical, light-colored form. Even taking into consideration the errors in the model, 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 those morphs that were not camouflaged with increased frequency.

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 coloration.

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 indistinquishable 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.

Environmental changes


Before the Industrial Revolution, the peppered moth was mostly found in a light gray form with little black speckled spots. The light-bodied moths were able to blend in with the light-colored 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-colored lichens and English trees, therefore, the light-colored 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 sulfur dioxide emissons, and the trees became covered with soot. This led to an increase in bird predation for light-colored moths, as they no longer blended in as well in their polluted ecosystem: indeed, their bodies now dramatically contrasted with the color of the bark. Dark-colored moths, on the other hand, were camouflaged very well by the blackened trees.

Although a majority of light-colored moths initially continued to be produced, most of them didn't survive, while the dark-colored 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-colored moths had risen noticeably, and by 1895, the percentage of dark-colored moths in the Manchester peppered moth population was reported at 98%, a dramatic change (by almost 1000%) from the original frequency. This evolved darkening in color as a result of industrialization has come to be known as industrial melanism as a result.

In modern times, due to 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, due to 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 industrialized regions.

Predation experiments


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, that typica morphs were preferentially taken. 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 that had been made of Kettlewell's experimental methods. Criticism and 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 due to 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."

Alternative hypotheses
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 mechansisms 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.

Phenotypic induction
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.

The first to try to prove this hypothesis was Hasebroek (1925), who 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. Further evidence, if it were needed, is likely to come from research into the biochemistry of melanism.

Criticism and controversy
In recent years, the use of the peppered moth as an example of evolution has come under attack by creationists and advocates of intelligent design, who allege that it is not reliable as evidence of evolution.

Critics have pointed out that the "peppered moth story" showed only microevolution, rather than the important macroevolutionary trend of speciation (e.g. Watchtower Bible and Tract Society, 1985). Biologists agree with this point, and accept that correlation between soot on tree trunks and observed melanism in the moths is not proof of the theory of evolution as a whole. However, many do not accept the supposed distinction between "microevolution" and "macroevolution" as being part of the modern evolutionary synthesis which equates the two, instead taking the view that the mechanisms of microevolution and macroevolution are the same, the only difference being of time and scale.

Another common criticism involves well-known pictures of moths resting on trunks, used in many textbooks. These photos were prepared (dead moths pinned to branches), which has been conflated into the idea that all the studies were staged, ignoring the point that professional photography to illustrate textbooks uses dead insects due to the considerable difficulty in getting good images of small, relatively fast moving, animals, and that the 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 no different than the "staged" photographs.

Furthermore, while some experiments did involve the pinning 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 test the idea of differential bird predation: the placing of dead moths was done so that scientists could control the location variables and watch the same sites to see if the behavior of birds differed depending on which sorts of moths were placed on which sorts of sites.

The methodology of Bernard Kettlewell's classic study was questioned in a review by the biologist Jerry Coyne in Nature of Michael Majerus's 1998 book Melanism: Evolution in Action which includes a critique of Kettlewell's experiment, matching a similar 1998 analysis by Sargent et al. Coyne stated that the most serious problem found by Majerus was that only two peppered moths had been found on tree trunks. He also noted 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. At the beginning of his second paragraph on the peppered moths, Majerus emphases that the wealth of additional data obtained since Kettlewell's initial predation papers does not undermine the basic qualitative deductions 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 cites 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.

The review was subsequently picked up by journalist Robert Matthews, who wrote an article for The Sunday Telegraph, March 14, 1999, claiming that "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". Majerus regarded this view as surprising, and not one that would be shared by those involved in the field. He noted numerous scientific inaccuracies, misquotations and misrepresentations in the article, but thought this was common in press reports. He stated that he had spoken to Matthews for over half an hour and had to explain many details as Matthews hadn't read the book, but "Even then, he got nearly everything wrong."

The story was taken up by creationists, and at a seminar presenting his wedge strategy on March 13, 1999, the leading intelligent design proponent Phillip E. Johnson asserted 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 May 24, 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 he 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 1980's 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 basically dishonest.

On November 27, 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, Judith Hooper's Of Moths and Men was published, expressing a similar argument to Wells about Kettlewell's experimental methodology. This was criticized in scientific journals (e.g., Coyne, B.C. Clarke, Grant). Majerus described it as "littered with errors, misrepresentations, misinterpretations and falsehoods".