SELECTION

in biology, the preferential survival and reproduction or preferential elimination of individuals with certain genotypes (genetic compositions), by means of natural or artificial controlling factors. The theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in 1858. They argued that species with useful adaptations to the environment are more likely to survive and produce progeny than are those with less useful adaptations, thereby increasing the frequency with which useful adaptations occur over the generations. The limited resources available in an environment promotes competition in which organisms of the same or different species struggle to survive. In the competition for food, space, and mates that occurs, the less well-adapted individuals must die or fail to reproduce, and those who are better adapted do survive and reproduce. In the absence of competition between organisms, natural selection may be due to purely environmental factors, such as inclement weather or seasonal variations. (See natural selection.) Artificial selection (or selective breeding) differs from natural selection in that heritable variations in a species are manipulated by humans through controlled breeding. The breeder attempts to isolate and propagate those genotypes that are responsible for a plant or animal's desired qualities in a suitable environment. These qualities are economically or aesthetically desirable to humans, rather than useful to the organism in its natural environment. In mass selection, a number of individuals chosen on the basis of appearance are mated; their progeny are further selected for the preferred characteristics, and the process is continued for as many generations as is desired. The choosing of breeding stock on the basis of ancestral reproductive ability and quality is known as pedigree selection. Progeny selection indicates choice of breeding stock on the basis of the performance or testing of their offspring or descendants. Family selection refers to mating of organisms from the same ancestral stock that are not directly related to each other. Pure-line selection involves selecting and breeding progeny from superior organisms for a number of generations until a pure line of organisms with only the desired characteristics has been established. Darwin also proposed a theory of sexual selection, in which females chose as mates the most attractive males; outstanding males thus helped generate more young than mediocre males.