gives the conditions where genetic drift and natural selection have approximately equal influence on the fate of allele frequencies. When 2 s is within an order of magnitude of the reciprocal of effective population size, an allele can be described as net neutral or nearly neutral since natural selection and genetic drift are approximately equal forces dictating the probability of fixation of an allele. Next, notice that multiplying both sides of equation 8.4 by 2Ne gives 4Nes = 1 as the condition where the processes of genetic drift and natural selection are equal. When 4Nes is much greater than one natural selection is the stronger process whereas when 4Nes is much less than one genetic drift is the stronger process.

Using more sophisticated mathematical techniques, Kimura (1962) showed that the probability of fixation for a new mutation in a finite population is

where p is the allele frequency (p = for a single mutation but in general p is assumed to be much less than one), Ne is the effective population size, and s is the selection coefficient assuming codominance. This equation is plotted in Fig. 8.5 along with the constant probability of fixation for a new mutation expected under neutral theory.

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