Science Fair Project Encyclopedia
Peppered moth evolution
The evolution of the peppered moth over the last two hundred years has been studied in detail. Evolution, more specifically microevolution is defined at the operational level as "a change in the frequency of an allele within a gene pool" (Dobzhansky, 1937); see population genetics. Since the genetics of how the morphology is controlled are known (see above), changes in phenotype frequency changes indicate such allele frequency changes.
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 a aberrant morph caused by a recurrrent 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.
(the following is figure 3, until permission can be granted (pun not intended), it will not be uploaded onto Wiki server) -->
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.
Hypothetical models of evolution
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.
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details