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Nora Guschwan Biology 182 H Essay Submitted: April 13, 2005 the Melanism Wars “The Trouble with Classical Examples of Evolutio

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Nora Guschwan Biology 182 H Essay Submitted: April 13, 2005 the Melanism Wars “The Trouble with Classical Examples of Evolutio 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.
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