Effects of selection and drift on the dynamics of finite populations : IV. Gene frequency dependent selection.

A genetic model of a two allelic locus involving gene frequency dependent selection with overdominance or underdominance was investigated with regard to the probability of and the expected time to fixation of an allele in the face of stochastic variations arising from finite population size. Results showed the following: 1. If the probability of fixation is larger than for no selection, the case of fequency dependent selection with overdominance at equilibrium gave rise to the highest probability of fixation. If, however, the probability of fixation is less than for no selection, the case of frequency dependent selection and underdominance at equilibrium gives rise to the highest probability of fixation. 2. The expected time to fixation is largest for the case of overdominance at equilibrium and smallest for the case of underdominance at equilibrium if 0.2< p < 0.8. 3. For equilibrium gene frequencies (p) above 0.8 or below 0.2 there was acceleration in the time to fixation. This acceleration occurred over a wide range of initial gene frequencies that includes the p value.