By application of the neutral model of phenotypic evolution, quantitative estimates of the rate of input of genetic variance by polygenic mutation can be extracted from divergence experiments as well as from the response of an inbred base population to selection. The analytical methods are illustrated through a survey of data on a diversity of organisms including Drosophila, Tribolium, mice, and several crop species. The mutational rate of introduction of genetic variance (Vm) scaled by the environmental variance (VE) is shown to vary between populations, species, and characters with a range of approximately 10−4 to 5 × 10−2. Vm/VE for Drosophila viability is somewhat below this range, while hybrid dysgenesis may temporarily inflate Vm/VE beyond 10−1. Potential sources of bias and error in the estimation of Vm are discussed, as are the practical implications of the observed limits to Vm/VE for projecting the long-term response to selection and for testing adaptational hypotheses.
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