1. The combined effect of the causes and consequences of genetic factors that determine the average number of progeny of a species that attain sexual maturity; phenotypes that are lethal early in life (Tay-Sachs disease), that cause sterility (Turner syndrome), or that produce sterile progeny are selected against. When selection is used of individual pedigrees, other factors, notably variance of the number of progeny and number that survive to maturity, are important considerations; in large populations, these factors even out and the mean only is of importance.

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