Nora Guschwan Biology 182 H Essay submitted: April 13, 2005

The Melanism Wars

“The trouble with classical examples of evolution is that they continue to evolve.” (Grant 1999).

The theory of evolution by natural selection has been a hotly debated topic since Charles

Darwin’s 1858 “Origin of Species”. The dialogue is scientific as well as spiritual and cultural.

Everyone has a vested interest in where life came from and how (and if) it evolves. What a person believes about this issue shapes their identity and sense of purpose. This quest for meaning that is the human existence has important implications in how we view the practice of science. As a culture, we often look to scientists to give us absolute explanations about the world. Many scientists even believe this is their job (Oreskes 2004). Assigning this pressure to the results of scientific investigation misses the point. Science is an ongoing process. While it provides explanations about the world around us, the nature of science requires its findings to always be open for continued study and scrutiny. This fluid quality is what allows us to gain ever-greater insights. The trials and tribulations of the peppered moth illustrate one scientific theory that has battled scientific and public opinion, and is at risk of extinction because of it.

The peppered moth, or Biston betularia, is one of thousands of moth species found in

England. It was effectively monomorphic throughout the U.K. until the middle of the nineteenth century. The typica variety is white with black spots. Around 1848, a melanic (or darkened) form began to show up around Manchester. By the close of the nineteenth century, this new black- winged form reached a frequency of up to 98% in some areas. The rate at which this phenotypic reversal happened was mind boggling, especially since B. betularia are univoltine. Alas, the longstanding question was born, what caused this to happen? Scientists have had a variety of theories from the beginning. In 1890, E.B. Poulton suggested that color might have an effect on thermal efficiency (Cook 2000). In 1920, the geneticist J.W.H. Harrison argued that melanism could be induced in adults if the larvae are fed chemically contaminated leaves (Grant 1999). However, what sparked the still ongoing peppered moth debate was the theory of entomologist J.W Tutt. In 1896 he suggested that typica was protected from avian predation due to its resemblance to the lichen cover of the trees it rested on

(Owen 1997). In manufacturing regions in England, the lichens were destroyed by pollution and tree surfaces were blackened by soot. The formerly camouflaged typica was now conspicuous to predators, paving the way for the inconspicuous melanic to flourish. Precious evidence for the theory of evolution by natural selection was beginning to emerge.

In the years that followed, the scientific milestone known as the modern synthesis took shape. The modern synthesis advanced evolutionary theory through incorporating genetics, systematics, and paleontology. Genetics, specifically particulate inheritance, provided a mechanism for Darwin’s theory of evolution. In a 1914 article by W. Bowater, he argues that the melanistic mode of inheritance for the peppered moth is controlled by a single, dominant gene

(Majerus 1998). In 1924, the Oxford geneticist J.B.S Haldane calculated the fitness advantage of the melanic required to account for its rapid spread. He discovered that carbonaria would have to be one and a half times as fit as its typica relative (Haldane 1924). This is in contrast to

Darwin’s idea that evolution only happens very slowly.

Armed with this information, the British ecologist H.B.D. Kettlewell set out to confirm

Tutt’s theory. While a graduate student at Oxford in the 1950s, he conducted a variety of experiments on the peppered moth. His objective was to determine if birds ate cryptic moths in their natural resting place, and if they did so selectively. His work included quantitative rankings of camouflage effectiveness (judged by the human eye) of pale and melanic peppered moths on various backgrounds, direct observations of bird predation on moths placed on tree trunks, and recapture rates of marked and released moths (Grant 1999). The predation experiments with

B.betularia were carried out in polluted (1953) and unpolluted (1955) woodlands. His results revealed that in the polluted environment near Birmingham, birds more heavily predated typica than carbonaria. The reverse was true in the unpolluted woodland in Dorset (Kettlewell 1973).

The complementary data from the contrary environments is the backbone of Kettlewell’s work

(Majerus 1998). This evolution story crossed over into the popular arena in 1959 when

Kettlewell published an article in Scientific American called “Darwin’s Missing Evidence”. The article was timely in that it came about just as the educational community was looking for ways to incorporate evolution into the pedagogy. Kettlewell’s peppered moth story became the classical, textbook example of evolution in action. It has been the quintessential educational model of evolution for over 40 years (Hagen 1999).

Not surprisingly, Kettlewell’s example has inspired criticism as well as further study of

B. betularia. It is generally agreed that any scientific theory held in high esteem should stand up against rigorous scrutiny (Sargent 1998, Grant and Howlett 1988). Kettlewell himself acknowledged that his methods did have some flaws. Many of these flaws were the basis of additional experiments done on the peppered moth.

Kettlewell’s first mark-release-recapture study was done in an aviary. The problem with his method here is that he released too many moths into this small area. The artificially high densities this created drove the predation rate up and probably made it less preferentially selective. Kettlewell observed that once a bird discovered one moth on a tree, that bird would spend time looking for more in the same location, thereby increasing the chance that an otherwise well-camouflaged moth would be eaten (Kettlewell 1973). This experiment did confirm that birds do eat peppered moths resting on trees, an important premise of the story. In

1966, Clarke and Sheppard did a similar study in a larger area using a much lower moth density.

Their results support Kettlewell’s hypothesis that differential bird predation is caused by differences in the degree of crypsis of the two peppered moth phenotypes (Clarke, et al. 1966).

In all of the mark-release-recapture experiments, Kettlewell released the moths onto tree trunks. He assumed this was their natural resting place. There is evidence that it is not the case.

Michael Majerus reports that in his many years of observing moths in the wild, the few he has actually seen resting in the daylight have not been in the same arboreal location (Majerus 1998).

Kauri Mikkola observed moths in captivity. He concluded that peppered moths hide on the underside of branches in the canopy (Mikkola 1984). Studies done by Grant and Howlett resulted in captive moths moving to rest close to where light entered their quarters (Grant and Howlett

1988). The bottom line of this conflicting data is that the natural resting places of peppered moths are not conclusively known. The effect this has on the validity of Kettlewell’s results is up for debate. Majerus believes this knowledge is “crucial” to assessing morph fitness. While the results of his 1986 pilot selection experiments (done with Howlett) qualitatively agreed with

Kettlewell’s results, he argues that fitness estimates that assume trunk resting are quantitatively incorrect (Howlett and Majerus 1987). Others believe any negative effect is off set by the fact that he was consistent throughout his investigations. Laurence Cook surmises that Kettlewell’s woodland experiments compared the fitness of different morphs on the same parts of trees in different areas, not on different parts of trees in the same area (Cook 1998).

Kettlewell also used a mixture of wild and lab bred moths in the woodland mark-release- recapture experiments. This was done to beef up the local population of the “less favored” moth in that area (Kettlewell 1973). This was a poor scientific choice because it could have introduced more, and unnecessary, variables into the equation. There could be differences in longevity or flight ability (Grant 1999). Bishop (1972) concluded that recapture rates were not significantly affected with regard to lab bred versus wild moths. Subsequent experiments often employed freshly killed specimens to alleviate any potential issues (for instance, Clarke and

Sheppard 1966).

Another point of contention in Kettlewell’s work is the time of day he released the moths.

He released them when the sun was already up. He did this to keep the moths from flying directly into his traps while it was still dark (Kettlewell 1973). It is well known that moths are night-fliers who rest by day. The consensus is that they will choose their resting spot just before daylight, so they are already still when their predators come out to feed (Grant 1999). The problem, then, with Kettlewell’s release time is that it could have significantly altered their behavior. They may have hastily chosen the first spot available to rest. Without further experimentation, it is not clear how and if this affects the validity of his hypothesis. Again, the problem may be minimized by the fact that Kettlewell was consistent in his methods. He was comparing predation rates based on crypsis and background. It would have been ideal to let the moths choose their resting spot at the time they normally would, but this doesn’t change the fact that the less camouflaged moths were eaten more than the better-camouflaged variety.

Kettlewell performed barrel experiments to show that melanic and typical peppered moths actively select resting sites with different backgrounds, the results suggested that typicals preferentially chose lighter backgrounds while melanics chose darker ones. He proposed a

“contract/conflict” mechanism for this in which moths would visually compare tufts of scales around their eyes to the surface under them. They would then choose the spot with the least contrast (Kettlewell 1973). He did not test this mechanism. Sargent (1968) did test the contrast/conflict mechanism. He painted the scales of the moths with contrasting colors to see how this would affect their choice. It didn’t. So, Sargent deduced that the moth’s resting site selection ability lies in their genetic make up, but provides no evidence. Kettlewell himself dismissed Sargent’s assertion as being irrelevant to the peppered moth because he used two different monomorphic species (Grant 1999). Grant and Howlett (1988) also tested this mechanism. Their results differed from Kettlewell’s and Sargent’s. Instead, their data corroborated Mikkola’s 1984 assertion that there is no morph-specific behavior related to resting site selection (Grant 1999). My impression is that the background available to them is what’s important. From an evolutionary perspective, isn’t this what drives phenotypic gene frequency?

Cook and Majerus contend that a sophisticated resting site selection mechanism would actually reduce the importance of differential visual selection in the maintenance of the polymorphism

(Majerus 1998).

Some of the most compelling evidence is not directly related to Kettlewell’s work. Clean

Air legislation was enacted in Britain in 1956 and in the United States in 1963. Subsequently, the frequency of melanic peppered moths declined at locations in both countries. While the rise in melanic frequency was not documented well, now the fall could be. In 1996, Grant, Owen, and

Clarke conducted a comparative study in Caldy Common, England and at the George Reserve in

Michigan. Caldy Common was selected because it had been continuously sampled and melanic decline had been documented since 1959 (Cook et al. 1986). The George Reserve was chosen because Owen had documented melanic frequencies there over 90% (Owen 1961). They concluded, “…..the changes in melanic frequencies in the Michigan population have paralleled in both directions the well-documented changes in England.” (Grant, et al. 1996). This was striking evidence for the peppered moth tale. They also postulated that the parallel genetic changes were the result of parallel causes. They back up this assertion by stating that three of the four evolutionary forces of classical population genetics can be ruled out. These are genetic drift, mutation, and migration. This leaves natural selection as the evolutionary force driving these parallel changes in allele frequencies at the locus for melanism (Grant, et al. 1996).

Another critical issue addressed in this paper is the importance of lichens. At Caldy

Commons, typica frequency increased well before the lichens returned to the trees. There are also vast differences in tree species in Michigan and England. What Grant, et al., discuss is that the environmental change common to both locations is improved air quality. Decrease in atmospheric SO2, an index of pollution, is correlated to a decrease in melanic phenotypes

(Clarke, et al. 1985, 1990). High levels of SO2 indicate high levels of airborne particles. These particles settle on surfaces as soot, which changes the reflectance of those surfaces. SO2 levels have been shown to affect the distribution of carbonaria (Lees, et al. 1973). This suggests that the importance of lichens, specifically, was incidental in Kettlewell’s work. It is the reflectance of the surface (which is affected by lichens) that may be more important in the resultant crypsis.

In spite of imperfections in Kettlewell’s methods, differential crypsis in peppered moths driven by avian predation remained an important example of evolution by natural selection for the forty years since Kettlewell’s initial work. That is until 1998.

Chronologically, this is the point where the debate gets heated. Michael Majerus’s book

Melanism: Evolution in Action was published in 1998. In it, he re-evaluates the peppered moth paradigm, 25 years after Kettlewell’s book (which includes a summary of his experiments). He critically assesses the evidence and discusses additional factors pertinent to the case, including

UV visual sensitivity of birds. Although he does plea for continuing work to be done, he concludes “My view of the rise and fall of the melanic peppered moth is that differential bird predation in more or less polluted regions, together with migration, are primarily responsible, almost to the exclusion of other factors.” (Majerus 1998).

In spite of this statement, a review of his book by Jerry A. Coyne conveys that Majerus does not stand by the peppered moth story and implies that because of his comments, the story should be “shelved” for now (Coyne 1998). Adding fuel to the fire, The Sunday Telegraph printed an article by Robert Matthews entitled “Scientists Pick Holes in Darwin Moth Theory” on March 14. 1999. The article alleges that evolution experts were abandoning the example due to irreparable “scientific blunders” in the initial experiments.

The controversy snowballed in 2002 when a book by Judith Hooper entitled “Of Moths and Men” was published. Here, Hooper accuses Kettlewell of fraud. The basis of her accusation is that there was a marked increase in his recapture rates during the polluted woodland predation experiments. She argues that this increase followed a letter received by Kettlewell from his mentor, E.B Ford. Pressured by the letter, wanting to please Ford, Hooper asserts that Kettlewell fudged his data. She implies that he changed his procedures “until he got the desired results”.

This would render his hypothesis unfalsifiable, and therefore unscientific (Hooper 2002). Bruce

Grant counters her position by saying that she misinterpreted Ford’s letter (Grant 2002). He also reveals that Kettlewell himself noted the changes in experimental design that could account for the sudden increase. He tripled the number of moths released. In his review of her book, Jerry

Coyne criticizes Hooper for negating reasonable factors that could explain recapture increases simply because she was able to rule out weather changes (Coyne 2002). I agree with Grant’s estimation that Judith Hooper is jumping to pseudo-scientific conclusions (Grant 2000). In her narrow-minded effort to discredit Kettlewell, she does not consider the fact 35 other field experiments (performed from 1966 – 1987) show a strong correlation between fitness and frequency of adult moths in the wild. This is consistent with the industrial melanism hypothesis

(Cook 2000, Mallett 2003).

To confound her lack of credibility, Hooper champions Ted Sargent. Ted Sargent and camp (including Craig Millar and David Lambert) promote the theory of induction, initially proposed by Harrison in 1920 (Sargent, et al. 1998, Lambert, et al. 1986). The problem with this theory is that it supports a Lamarckian view of inheritance. The mandate of Lamarckian inheritance is that traits acquired during an organism’s lifetime can be passed on to their offspring. While Sargent agrees that melanism in moths often exhibits a Mendelian mode of inheritance, he does not agree that it always arises as a mutation. This is where he proposes induction triggered by environmental change as a plausible alternative (Sargent, et al. 1998).

This mechanism has been discredited by the modern synthesis, and Ted Sargent offers no new evidence to the contrary. His 1998 paper cites late 19th/early 20th century work that attributed anomalous results in breeding experiments to induction, while it was those same breeding experiments that established the peppered moth’s Mendelian inheritance.

The controversy surrounding the peppered moth also attracted the attention of a creationist named Jonathan Wells. He wrote a book entitled “Icons of Evolution” published in

2000. He attacks the peppered moth as an icon of evolution by selectively citing the literature.

He exploits the small, often irrelevant, details that may not still hold and cites these as reasons to discard the whole story. One example is his criticism that lichens are not always present on trees.

Yet, as mentioned in this paper before, it is substrate color/reflectance, not presence or absence of lichens, that is critical (Padian, et al. 2002). What is particularly frightening about this book is that nowhere does it mention creationism (Grant 2000). Yet, a bit of internet investigation makes it obvious that Mr. Wells is a creationist whose agenda is to “bring down Darwinism”.

The story of B. betularia is not over. The work of H.B.D. Kettlewell laid the groundwork for a dynamic and ongoing quest for the truth about the role of the peppered moth as an exemplary model of evolution. There are many avenues further experimentation should take in order to enhance and refine this “modern paradigm”. Michael Majerus’s work in 2000 emphasizes the importance of understanding the UV vision of birds and moths when assessing crypsis. He also re-evaluates the effects of types of lichens in relationship to the still unknown arboreal resting location of moths. These are definite issues to address in future work.

Additionally, the physiology and population biology of peppered moths requires more comprehensive examination. Creed and Lees (1980) argue that since melanic frequency never achieved 100% in the polluted environments, there are factors other than visual selective predation maintaining the polymorphism. They suggest differences in pre-adult viability of the genotypes as a contributing factor. Others have suggested melanic heterozygote advantage

(Clarke and Sheppard 1966), as well as frequency-dependent selection (Bishop and Cook 1979).

In his book, Majerus (1998) suggests mating habits can have an affect on location density.

Brakefield and Liebert (1990) emphasize the importance of understanding migration of adult males in modeling selection and migration in the wild.

In this day and age, the most obvious need for further work is in the molecular genetics of the peppered moth. This type of data is necessary to fully understand melanic heterozygote versus homozygote fitness as well as to fully explain responses of polymorphisms to natural selection (True 2003). Gene flow resulting from migration could also be estimated by using molecular techniques to assess genetic subdivision among populations (Grant 1999). It would be ideal for a group of scientists to lay out an “experimental plan”, like a medical treatment plan, to systematically resolve the issues surrounding melanism in Biston betularia. They could address all of the conflicting ideas and new questions that have come up over the years. This would require an objective, altruistic panel of scientists who are concerned with the truth, not with promoting their name. This particular example could really benefit from this process because evidence in natural populations is hard to come by. There are so many variables to consider in the wild. Maybe, with a pool of great funds and great minds, a new level of scientific research and evidence could be achieved. Optimistic, I know.

The unfortunate result of this entire hullabaloo, scientific and popular, is that industrial melanism in Biston betularia as a classic example of evolution by natural selection has been removed from many textbooks. This is an unfortunate, misplaced meeting of science and popular culture. Instead of burying this icon, we should be expanding the textbook explanation. Teachers could review Kettlewell’s classical story with their students, then open up a dialogue about how it may be oversimplified in light of subsequent studies and evidence. Using this approach, teachers could convey the many complexities inherent to the scientific process (Rudge 2000).

This would require a change in educational practice from a dogmatic pedagogy to one that is more fluid and discipline oriented. This story, in all of its splendor, could provide a wonderful opportunity to teach young minds how to think. Given our linear thought process which hungers for fixed, fast, and easy answers, this shift would be revolutionary.

In conclusion, I agree with Laurence Cook who said, “….The Biston story continues to provide an exceptional opportunity to analyse a pattern of selection. It should be pursued, along with study of other species with related but different responses to environmental change.” (Cook 2000). The peppered moth story is an excellent example of microevolution. It is not the end all, be all, pillar of proof for evolutionary theory, nor should this be expected of it.

We should celebrate the work it has inspired, not trash it because of a few inconsistencies and dissenting opinions. BIBLIOGRAPHY

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