The probability of fixation of a gene in a finite population when selection coefficients vary randomly from one generation to the next has been investigated. Some earlier equations of Ohta (1972) are corrected and the numerical values recalculated. When the variance of the selection coefficient is large (2NVt >> 1), both advantageous and disadvantageous alleles have probabilities of fixation nearly 0.5, which may differ substantially from the initial gene frequency. When 2NVt is small, the probability of fixation is governed by the geometric mean over time of the fitness of the allele. Variation of the selection coefficient can therefore greatly increase the probability of fixation of a single mutant above its initial frequency, when the mutant is neutral or disadvantageous in its (geometric) mean fitness.
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