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Unformatted text preview: of the population, it must be greater than , the mean fitness of a alleles. Thus the allele with the higher mean fitness increases in frequency. It should be noted that the fitnesses W A/A , W A/a , and W a/a may be expressed as absolute probabilities of sur vival and absolute reproduction rates, or they may all be rescaled relative to one of the fitnesses, which is given the standard value of 1.0. This rescaling has absolutely no effect on the formula for p 9 , because it cancels out in the numerator and denominator. W a recently entered a population of homozygotes a/a. At first, the change in frequency is very small because p is still close to 0. Then it accelerates as A becomes more frequent, but it slows down again as A takes over and a becomes very rare ( q gets close to 0). This is precisely what is expected from a selection process. When most of the population is of one type, there is nothing to select. For change by natural selection to occur, there must be genetic variation; the more variation, the faster the process. One consequence of the dynamics shown in Figure 1911 is that it is extremely difficult to significantly re duce the frequency of an allele that is already rare in a...
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.
 Spring '08
 JOHNSON
 Biology

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