NATURAL SELECTION

From Big Medical Encyclopedia

NATURAL SELECTION — process of selective survival of live organisms, main driving factor of organic evolution. The doctrine about E. the lake is created in the middle of 19 century by Ch. Darwin and is a basis of the modern theory of evolution (see. Theory of evolution ).

From potential ability of reproduction inherent in all living beings in a geometrical progression («pressure of life») and impossibility of implementation of this ability in the nature because of coexistence of many types in the biosphere limited by the sizes and resources inevitably there is «a fight for existence» — the diverse relations between the existing organisms (from direct fight before mutual assistance) connected with survival and leaving of posterity. The mutational processes which are constantly going in the nature (see. Mutation ) and genetic combination theory during the crossing of individuals define a genetic and phenotypical variety of individuals in limits populations (see). «Pressure of life» in combination with heterogeneity of individuals in any population also is premises E. lake. A selection response is elimination from reproduction of one individuals (or the whole groups of individuals — in case of group E. lake) and preferential reproduction of others differing in a «useful» way in these conditions from other individuals. If these differences are hereditary, based on mutational variability (see), collecting in a lineage, they can lead to emergence of new signs, properties and morphogenesis, up to emergence of new individuals. One mutational variability without natural selection can lead only to loss of already available fitness as a result of uncontrolled accumulation of mutations.

Allocate two main forms E. lake: stabilizing E. lakes, at Krom in constant and homogeneous conditions are destroyed all considerable deviations from average norm, and driving E. lakes, at Krom in the course of evolution are picked up certain deviations from former norm under the changing living conditions. In pure form any of these forms of selection does not meet in the nature. Strict proof of action E. the lake as the factor of evolution leading to transformation of types was received only in 20 century. In particular, emergence of «an industrial melanizm» — dark coloring of light forms of butterflies in the regions of Europe with intensive industrial pollution of the environment is studied before. E. the island gradually led to replacement before of eurysynusic light forms by mutant dark forms. The data on the directions and intensity obtained for the last decades E. lakes showed, on the one hand, a possibility of action of selection even on the smallest biochemical features (the selection value of the proteinaceous polymorphism characteristic of all types studied in this respect), and with another — on very high selection intensity on a nek-eye to signs (e.g., resistance of insects to insecticides). Sometimes action of selection supports a sign, obviously harmful to an organism, or property if it is connected with other sign having important adaptive value. E.g., in some regions of the Mediterranean and tropical Africa it is eurysynusic drepanocytic anemia (see) — the hereditary disease of blood which is shown at individuals, homozygous on this gene. Heterozygotes differ in immunity to malaria therefore E. the lake maintains very high concentration of this semi-legal gene.

As a result of synthesis classical Darwinism (see) and geneticists (see), occurred at the beginning of 20 century, there were clear many other essential parties of action E. lake. So, e.g., the impossibility of selection on the one and only sign as in development each gene is connected with a great number of others was investigated and selection on one gene inevitably causes automatically occurring «sootbor» on the whole complex of genes. It can result in unexpected results and a message to change of the main initial direction of selection.

E. the lake will transform populations according to conditions of the environment and has no definite purpose. The main distinction between processes natural consists in it and artificial selection (see). Artificial selection aims at fixing of the necessary forms of the signs satisfying needs of the person, and E. the lake — the signs corresponding to conditions of the environment. At all stages biol, evolutions of ancestors of the person E. the island played the leading role. However after emergence of society the role of factors biol, evolutions qualitatively changed. In particular, influence E was sharply weakened. lake. Effects of such release of pressure of selection are not quite clear for mankind since it is difficult to estimate objectively effects of accumulation of the increasing number of various hereditary deviations.


Bibliography: Darwin H. Collected works, the lane with English, t. 3, page 253, M. — L., 1939; M and y r E. Populations, types and evolution, the lane with English, M., 1974, bibliogr.; Timofeev-Resovsky N. V., Baneberries N.N. and Yablokov A. V. Short sketch of the theory of evolution, M. 1969; Sheppard F. M. Natural selection and heredity, the lane with English, M., 1970, bibliogr.; Shmalgauzen I. I. Factors of evolution, the Theory of the stabilizing selection, M., 1968, bibliogr.; Yablokov A. V. and Yusufov A. G. Theory of evolution, M., 1976.

A. V. Yablokov.

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