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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 (e.g., Tay-Sachs disease), that cause sterility (e.g., Turner's syndrome), or that produce sterile progeny are selected against. When s. 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. [L. se-ligo, to separate, select, fr. se, apart, + lego, to pick out]
artificial s.interference by man with natural s. by purposeful breeding of animals or plants of specific genotype or phenotype to produce a strain with desired characteristics; e.g., breeding of dairy cattle for high milk production.
medical s.preservation, by medical care and treatment, of individuals of pathologic genotypes who would not otherwise reproduce, thus tending to increase the frequency of pathologic genes in the population; conversely, reduction of the frequency of pathologic genes by preventing reproduction of individuals of specified genotype by surgical sterilization or other means.
natural s."survival of the fittest," the principle that in nature those individuals best able to adapt to their environment will survive and reproduce, while those less able will die without progeny; and the genes carried by the survivors will increase in frequency. This principle is heuristic rather than rigorous since it cannot be tested, the outcome being tautologous with the empirical definition of fitness.
sexual s.a form of natural s. in which, according to Darwin's theory, the male or female is attracted by certain characteristics, form, color, behavior, etc., in the opposite sex; thus modifications of a special nature are brought about in the species.
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