Phenotypic Evolution by Neutral Mutation

A general model is developed for predicting the genetic variance within populations and the rate of divergence of population mean phenotypes for quantitative traits under the joint operation of random sampling drift and mutation in the absence of selection. In addition to incorporating the dominance effects of mutant alleles, the model yields some insight into the effects of linkage and the mating system on the mutational production of quantitative‐genetic variation. Despite these additional and potentially serious complications, it is found that, for small populations, the simple predictions obtained by previous investigators using additive‐genetic models hold reasonably well. Even after accounting for dominance and linkage, the equilibrium level of genetic variance is unlikely to be much less than 2NVm or to be more than 4NVm, where N is the effective population size and Vm is the new variance from mutation appearing each generation. The rate of increase of the between‐line variance per generation ultimately equals 2Vm regardless of population size, although the time to attain the asymptotic rate is proportional to N. Expressions are presented for the rate of approach to the equilibrium level of genetic variance and for the expected variance of the within‐population and between‐population genetic variances. The relevance of the derived model, which amounts to a generalization of the neutral theory to the phenotypic level, is discussed in the context of the detection of natural selection, the maintenance of pure lines for biomedical and agricultural purposes, the development of genetic conservation programs, and the design of indices of morphological distance between species.