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Figure 8.5 The probability of eventual fixation for a new mutation under the neutral and nearly neutral theories. Under the nearly neutral theory the probability of fixation depends on the balance between natural selection and genetic drift, expressed in the product of the effective population size and the selection coefficient (Nes). When negative selection operates against a deleterious allele, the selection coefficient and Nes are negative. Values of Nes near 0 yield a fixation probability close to that predicted by neutral theory. Only when the absolute value of Nes is large does natural selection exclusively determine the probability of fixation. Neutral theory assumes that neutral mutations are not influenced by selection and have a constant probability of fixation dictated by the effective population size. In this example the initial allele frequency is 0.001, or the frequency of a new mutation at a diploid locus in a population of 500. After Ohta (1992).

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