selection(redirected from Directional selection)
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Related to Directional selection: Stabilizing selection
concept of evolution developed in the mid-19th cent. by Charles Robert Darwin. Darwin's meticulously documented observations led him to question the then current belief in special creation of each species.
..... Click the link for more information. , the mechanism of natural selection is considered of major importance in the process of evolutionevolution,
concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution.
..... Click the link for more information. . Popular formulations sometimes envisage a struggle for existence in which direct competition for mates or for various factors in the environment (e.g., food, water, and suitable space) counteracts the tendency toward overproduction of plants and animals resulting from the process of reproduction. But there are diverse ways other than direct struggle through which those organisms better adapted to the environment can survive and reproduce more successfully than those less fitted. A special form of natural selection, sexual selection, is also stressed in Darwinism. It attempts to account for secondary sexual characteristics that are not necessarily valuable in the struggle for existence. It assumes that the female selects as a mate one having the most highly developed of such characteristics, e.g., elaborate plumage or superior song, thereby perpetuating those characteristics. However, this interpretation is now questioned by many scientists. Artificial selection, the selection by humans of individuals best suited for their purposes, is common in plant and animal breeding.
in animal breeding, a form of artificial (methodical) selection; the choice of the most economically valuable animals for breeding. Along with the matching of parents that have been evaluated for the quality of their offspring and the proper rearing of the young, selection is an important means of creating and improving livestock breeds. In breeding work, individual selection based on comprehensive evaluation of animals, or boni-tation, according to their individual (phenotypic) and hereditary (genotypic) qualities, is most effective.
The basis of selection is genetic variation, which makes it possible to obtain desirable combinations of characteristics and to fix them in the offspring. The accumulation of beneficial qualities in the process of selection leads to improvement of breeds and the creation of new forms. In selection the body of the animal must be considered a single whole, and it is important to bear in mind C. Darwin’s principle of correlational variability in the development of separate parts of the body. In other words, selection for one characteristic often affects other related characteristics. Selection over a number of generations for one characteristic, for example, for external appearance or productivity, leads, as a rule, to the worsening of other features, to a general weakening of agricultural animals, or to various functional disorders.
The effectiveness of selection depends on the size of the animal population and its area of distribution (they must be sufficient), the fertility and early maturation of animals (rapidity of succession of generations), the nature of inheritance of characteristics, the variability of characteristics, the presence of correlative bonds between characteristics, and the intensiveness and direction of selection. (The higher the percentage of discarded animals in the herd, the better the part that remains, that is, the more rapidly does the herd improve.) A general index of the effectiveness of selection is the ratio of the index of the superiority of offspring of parents selected for breeding over the average population of the herd to the index of superiority of the parents over the same average.
E. IA. BORISENKO
in plant growing, the segregation of the best plants according to previously determined economic value and the segregation of the best seed material for subsequent reproduction. Selection is one of the principal methods of developing varieties of agricultural plants. The selection of plants and seeds usually takes into consideration such factors as potential yield and resistance to disease and pests. Two principal types of plant selection are used in the USSR: mass and individual.
In mass selection, a large number of homotypic plants that have a number of superior features are segregated. They are threshed together, and their seeds are sown in a single plot. This kind of selection is called single mass selection; if it is repeated over a number of generations, it is called repeated mass selection. Mass selection is simple and is widely used with cross-pollinating crops. This type of selection has some disadvantages. It is not possible to identify selected plants according to their offspring or to separate the most valuable forms from the population.
In individual selection, as in mass selection, superior plants are selected, but they are threshed separately. Their seeds are sown on separate plots. Thus, the parent plants can be identified according to their offspring. The offspring of inferior plants are discarded. The number of parent (elite) plants is usually from several hundred to 2,000 or 3,000. Individual selection may also be single or repeated. (See alsoARTIFICIAL SELECTION.)
REFERENCESObshchaia selektsiia i semenovodstvo polevykh kul’tur, 3rd ed. Moscow, 1958.
Pustovoit, V. S. Izbrannye trudy. Moscow, 1966.
Guliaev, G. V., and Iu. L. Guzhov. Selektsiia i semenovodstvo polevykh kul’tur. Moscow, 1972.
G. V. GULIAEV