Genetic Variability and Rate of Gene Substitution in a Finite Population under Mutation and Fluctuating Selection

By using a numerical method of solving stochastic difference equations, the level of genetic variability maintained in a finite population and the rate of gene substitution under several models of fluctuating selection intensities were studied. It is shown that mutation and random genetic drift both play an important role in determining genetic variability and the rate of gene substitution. Compared with the case of neutral mutations, the fluctuation of selection intensity caused by temporal and spatial heterogeneity of environments generally increases the rate of gene substitution, but the level of genetic variability may be increased or decreased, depending upon the model and the parameters used. Although such a type of selection per se can not be ruled out, when mutation is taken into account, it is difficult to explain both the observed amount of genetic variability and the rough constancy of evolutionary rate within a framework of fluctuating selection models.