Changes in Variance Induced by Random Genetic Drift

We noted previously that random genetic drift leads inevitably to the loss of alleles within populations and to the fixation of alternate alleles by different populations. These conclusions extend logically to quantitative characters. We expect the within-population variance of small populations to decline and the mean phenotypes of isolated populations to diverge gradually. There are some interesting surprises however. Throughout this chapter, we will continue to assume that selection is a negligible evolutionary force. We will consider in turn the dynamics of the withinand between-population variances, initially under the assumption that these are due entirely to genetic properties of the base population. Then the covariance between relatives in inbred populations will be considered. The role of mutation will be taken up at the end of the chapter.