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natural selectionthe process by which evolution takes place, involving the SURVIVAL OF THE FITTEST. The theory of evolution was proposed by Charles DARWIN and Alfred Russel Wallace in 1858 and was based on the observation that any characteristic found within a population exhibits variability. This means that some individuals will, as a result, have a survival advantage: they are more ‘fit’ for survival (and therefore more likely to reproduce and pass on their genes to the next generation) in the current environment. Thus, if skin pigmentation varies within a population living in equatorial regions, individuals with dark skin genes (which confer protection) are more likely to live to reproduce than those with lighter skin genes; and obversely in northern latitudes, where lighter skin maximizes the benefits of lower levels of sunshine. see also EVOLUTIONARY THEORY.
the fundamental moving force in the evolution of living organisms. Several British naturalists arrived at the idea of natural selection independently and almost simultaneously: W. Wells in 1813, P. Metthew in 1831, E. Blyth in 1835 and 1837, A. Wallace in 1858, and C. Darwin in 1858 and 1859. However, only Darwin was able to reveal the significance of this phenomenon as the principal factor in evolution and created the theory of natural selection. Contrary to artificial selection practiced by man, natural selection is conditioned by the influence of the environment on organisms. According to Darwin, natural selection is the survival of the fittest organisms, as a result of which evolution occurs on the basis of an undetermined (not corresponding to the influence of the environment) genetic variation in a number of generations.
Natural selection may be undergone not only by individual organisms but also by groups (varieties, races). The Soviet biologist I. I. Shmal’gauzen in 1946 developed the idea of group selection—the survival of populations, species, genera, families, orders, and so forth. But since group selection oc-curs on the basis of the survival of organisms that make up the groups, individual natural selection, the selection of the best adapted individuals, plays a leading role in evolution. A continuous mutation process, one that changes the genotypes, and free crossbreeding ensure the genetic variety of populations. Mutations and their combinations, which are manifested in the phenotype, condition the phenotypical variety of organisms (undetermined variability, according to Darwin). As a result, individuals of a given population react differently even to identical factors of the environment. Biological variations of characteristics in individuals in the population and high reproductive rates, which lead to a deficiency in vital resources—food, shelter—serve as preconditions for the struggle for existence in the course of which some individuals perish or are eliminated and some survive and are selected. Thus, natural selection can occur only in the presence of mutational variability;which creates the material for selection and constitutes the principal but not the only factor in evolution. The more intense the struggle for existence, the more intense the elimination (the death of individuals or groups of organisms) and the more strict the natural selection. But environmental changes that are too abrupt produce mass destruction—nonselective elimination, in which, just as in the absence of destruction, there can be no selection. Natural selection operates only when there is selective elimination—the destruction of less-fit individuals.The individuals that have survived natural selection transmit to their offspring their inherited characteristics (their geno-types), which is what ensures the possibility of the adaptive development of the next generation: natural selection operates according to phenotypes, but genotypes are selected. The significance of natural selection is not in survival as such but in the fact that the surviving individuals leave offspring.
Natural selection, which also determines the direction of evolution, depends not only on environmental changes but also on the character of the inherited variations, which, being better adaptable to the changed conditions of existence, are subject to the process of selection at a given moment. For this reason, in populations of the same species, even under very similar conditions, different variations may be selected, which leads to different directions of natural selection. At the same time, no matter how often nonadaptive variations may arise under any given conditions of variation, they will be eliminated and will exert no influence on the direction of selection. Thus, variability alone cannot determine the direction of evolution.
Natural selection emerges in two basic forms—stabilizing and evolutionary. Stabilizing selection, which was discovered by Shmal’gauzen, is observed among organisms of a given group under constant conditions of existence. In this event, all new mutations are harmful, since they disrupt the adaptation to the environment that has been created in the course of the preceding evolution of the group. New adaptations do not develop, but the adaptive norm that has already been achieved is maintained. The evolutionary form of natural selection, which was discovered by Darwin, is manifested when there are environmental changes. Then an inherited variation that coincides with the direction of a change in the conditions of existence is picked up by natural selection. In the long run, as a result of the destruction of individuals that do not have the advantage in a new living environment, a useful variation is gradually distributed throughout a population. If selection picks up one variation that is most advantageous under any given conditions, a population reorganizes as a single whole. Sometimes, how-ever, there occurs selection of several qualitatively different variations, which are approximately identically adaptive in relation to the same environmental factor. In such a case, several evolutionary directions arise in the same population, and so-called disruptive selection takes place. In nature both forms of natural selection coexist at all times. One may speak only of the predominance of evolutionary or stabilizing selection at any given stage in the evolution of a group under investigation. Stabilizing selection preserves the traits that have adaptive significance under certain conditions of existence; evolutionary selection “creates” new adaptations.
Owing to natural selection, any population has a known level of adaptability to the environment, which makes it possible for the organisms making up this population to survive the struggle for existence. In this, the supportive role of natural selection is expressed. But the level of adaptability supported by natural selection ensures the survival of individuals under those conditions to which they are adapted. For this reason, the distribution of organisms within the geographical range of a given species can be uneven: they survive under more suitable conditions. Thus, natural selection also influences the geographic distribution of organisms: dense groupings arise in a more favorable environment, while a less favorable one remains uninhabited. This expresses the distributive role of natural selection, which determines the population structure of a species.
New adaptations appear only as a result of natural selection. Mutational variability without natural selection (the uncontrolled accumulation of mutations according to Shmal’-gauzen) leads to the loss of adaptability. In the course of natural selection, variability itself is transformed: complementing each hereditary deviation that gives even partial advantage when there is environmental change, natural selection picks up all mutations of the genes that increase the adaptive significance of a given inherited variation and decrease its nonadaptive manifestations. In the process of mutation of the genes causing the modification and of the combination of mutations, which proceeds under the control of natural selection (only individuals who have undergone natural selection crossbreed), the new mutant gene, which was usually originally recessive, becomes dominant. In conformity with the adaptation that has newly emerged owing to correlative variations, the entire organism is reorganized. Parallel to this, as a result of the predominant elimination of individuals lacking the new adaptation, the entire population is transformed. The creative role of natural selection, which determines progressive evolution, is expressed in the creation of new adaptations, in the evolutionary reorganization of organisms, and in the transformation of populations, which leads to speciation. It is precisely this, the most important result of the action of natural selection, that also conditions its significance as the moving factor in evolution—a significance of which Darwin wrote: “I can see no limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life”(Soch., vol. 3, Moscow-Leningrad, 1939, p. 651).
Natural selection is sometimes compared to a sieve through which only pebbles of a certain size may pass. This analogy, however, is not accurate, since the “pebbles” that pass through the “sieve” of selection may change before the next “straining.” For this reason, views that ascribe only an eliminative role to selection, lead inescapably to acknowledgment of the Lamarckian principle of the inheritance of acquired characteristics (rejected by modern biology) and to an idealistic concept of the evolution of living beings.
REFERENCESHolden, J. Faktory evoliutsii. Moscow-Leningrad, 1935. (Translated from English.)
Darwin, C. Sochineniia, vol. 3. Moscow, 1939.
Simpson, J. Tempy i formy evoliutsii. Moscow, 1948. (Translated from English.)
Shmal’gauzen, I. I. Faktory evoliutsii, 2nd ed. Moscow, 1968.
Shmal’gauzen, I. \.Problemy darvinizma, 2nded. Leningrad, 1969.
A. A. PARAMONOV and A. S. SEVERTSOV