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artificial selection[¦ärd·ə¦fish·əl si′lek·shən]
selection of the economically most valuable animals and plants of any breed or variety and their use for breeding purposes.
The term “artificial selection” was introduced in 1859 by C. Darwin, who formulated the theory of artificial selection and showed that it is the main factor responsible for the origin and subsequent evolution of cultivated plants and domestic animals. Darwin demonstrated that every group of varieties or breeds and species of cultivated plants and domestic animals originated from one or several species of wild ancestors. Thus, all domestic pigeons are descended from the wild rock dove, all chicken breeds from the wild red jungle fowl, and all cabbage varieties from two or three related wild forms. The fact that a breed or variety shows great changes in economically useful characters is proof that its development was influenced by man.
A distinction is made between unconscious and methodical artificial selection. The concept of unconscious artificial selection was introduced by Darwin. Primitive herders and farmers tried to preserve the most valuable animal and plant specimens and obtain offspring from them. The preservation of the best animals from generation to generation guaranteed the reproduction of a herd, and the sowing of the best seeds made a good harvest more certain. Selection automatically preserved and spread in a breed or variety all the mutations that increased the economically valuable properties of organisms or weakened the injurious characters (from man’s point of view). At the same time, the carriers of abnormalities harmful to a breed or variety were inevitably removed in the process of elimination (destruction) of the less valuable individuals.
Therefore, unconscious artificial selection is similar to natural selection in both the mechanism of action (preservation of the more adapted forms) and in the result (adaptation of organisms to all the environmental conditions, including in this case man’s activity). Unconscious artificial selection is sometimes a side effect of biotechnological measures or genetic experiments. According to the World Health Organization (WHO), by 1963, without any effort by man to achieve this result, 47 insect species became resistant to DDT owing to the widespread use of toxic chemicals on insect crop pests and the ensuing selection for resistance to the poisons.
Methodical artificial selection began to be used in Europe in the second half of the 18th century because of the intensification of agriculture. It differs from unconscious artificial selection in being deliberate. (Individuals are singled out for their possession of a particular character or set of characters.) Methodical selection is the principal method used to breed plant varieties and breeds of domestic animals. Mass and individual methodical selection are used. Mass selection involves the rejection of all individuals who do not meet the standard set for a breed or variety. As a result, despite the constant ongoing mutation process, the group of organisms that is subjected to artificial selection is maintained at a prescribed economic level. However, the improvement of the selected characters usually takes place slowly. More effective is individual methodical selection whereby each parental form is judged not only by its own properties but also by its ability to transmit them to its offspring. Individual artificial selection entails, besides selection of the more valuable individuals, that is, selection proper, the matching of parental pairs, which in animal breeding is accompanied by an evaluation of the parents based on their offspring, thus helping to control the genetic properties of the parents.
Since most new mutations are recessive, the most effective method of quickly fixing in offspring the valuable properties of their parents is inbreeding, that is, the matching and mating of closely related individuals. This also ensures the homozygosity of the genes that determine the characters for which selection is undertaken. In inbreeding, however, other recessive alleles also inevitably become homozygous, causing undesirable characters to be expressed in the phenotype. In addition, high homozygosity reduces genotypic variety and brings the genetic state of breeds and varieties to that of a pure strain, thereby decreasing the effectiveness of selection. To prevent the harmful aftereffects of inbreeding, use is made of outbreeding, that is, mating of unrelated individuals, which increases the heterozygosity of organisms for recessive alleles. Since with individual methodical, artificial selection the best organisms are selected in each generation that persistently transmit their properties to the offspring, the characters for which selection is undertaken (based on the same genetic mechanism as in unconscious selection) are strengthened, but the process takes place much more quickly. For example, V. S. Pustovoit increased the oil content of sunflower seeds by almost 20 percent in 23 generations. (The work was carried out from 1940 to 1963.) The preferential development of some character based on selection is related to other characters of the organism through correlative variability. As a rule, the more extensively a breed (variety) is transformed and the better adapted it is to man’s needs, the worse its adaptation to the environment. Therefore, it is necessary to use biotechnological measures to create artificial habitats for domestic animals and cultivated plants that enable them to develop in optimum fashion, thus promoting a fuller expression of genetically determined properties of a breed (variety) in the phenotype and increasing the effectiveness of artificial selection.
REFERENCESDarwin, C. Soch.,vols. 3–4. Moscow-Leningrad, 1939–51.
Lobashev, M. E., K. V. Vatti, and M. M. Tikhomirova. Genelika sosnovami selektsii. Leningrad, 1970.
Williams, W. Geneticheskie osnovy i selektsiia rastenii. Moscow, 1968. (Translated from English.)
A. S. SEVERTSOV