hand, standard theory (Kimura, 1983) shows that the fixation probability of a mutation to A is e^s times that for a mutation to a, where S = 2 N_gs is twice the ratio of the power of selection to the power of random genetic drift (1/N_g). Because the population-level rate of transition from one allelic type to another is equal to the product of the mutation rate and the fixation rate, the ratio of probabilities of being in states A versus a at selection-mutation-drift equilibrium is simply me^s (Fig. 5.1). This simple expression leads to two general conclusions: (i) regardless of the strength of selection, if 1/N_g >> |s| the population will be driven to a state expected under mutation pressure alone; and (ii) the equilibrium composition of a population depends not on the absolute power of mutation, but on the relative rates of forward and reverse mutations (the mutation bias).

FIGURE 5.1 The long-term probability that an allele residing at a biallelic locus will be of the selectively advantageous type, given a selective advantage s, an effective population number of gene copies of N_g, and a mutation rate to the beneficial allele m times that in the reverse direction. The lowest curve (2 N_gs = 0.0) denotes neutrality, whereas the upper line (2N_gs = ∞) denotes an effectively infinite population, such that genetic drift is a negligible force.