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RISE AND FALL OF PHENOTYPE FREQUENCY Melanism has appeared in the European and North American 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 . This section considers Britain and Europe first, then North America, then Japan. 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 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 . Notably, Bernard Kettlewell conducted a national survey in 1956, and Bruce Grant conducted a similar one in the early 1996s. The results are shown in figure 3, 1956 on the left. {Link without Title} The source publication for these maps: Grant, B. S., Cook, A. D., Clarke, C. A., and Owen, D. F. 1998. Geographic and temporal variation in the incidence of melanism in peppered moth populations in America and Britain. Journal of Heredity 89:465-471. 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. MECHANISMS OF EVOLUTION Evolution in the wild is caused by two mechanisms; Natural Selection ; and Genetic Drift . J.B.S. Haldane in 1924 calculated using a simple General Selection Model 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, even taking into consideration the errors in the model. 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 were not camouflaged. |
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