Darwinism(redirected from Darwinian Theory)
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Darwinism,concept 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. developed in the mid-19th cent. by Charles Robert DarwinDarwin, Charles Robert,
1809–82, English naturalist, b. Shrewsbury; grandson of Erasmus Darwin and of Josiah Wedgwood. He firmly established the theory of organic evolution known as Darwinism.
..... Click the link for more information. . Darwin's meticulously documented observations led him to question the then current belief in special creation of each species. After years of studying and correlating the voluminous notes he had made as naturalist on H.M.S. Beagle, he was prompted by the submission (1858) of an almost identical theory by A. R. WallaceWallace, Alfred Russel,
1823–1913, English naturalist. From his study of comparative biology in Brazil and in the East Indies, he evolved a concept of evolution similar to that of Charles Darwin.
..... Click the link for more information. to present his evidence for the descent of all life from a common ancestral origin; his monumental Origin of Species was published in 1859. Darwin observed (as had Malthus) that although all organisms tend to reproduce in a geometrically increasing ratio, the numbers of a given species remain more or less constant. From this he deduced that there is a continuing struggle for existence, for survival. He pointed out the existence of variations—differences among members of the same species—and suggested that the variations that prove helpful to a plant or an animal in its struggle for existence better enable it to survive and reproduce. These favorable variations are thus transmitted to the offspring of the survivors and spread to the entire species over successive generations. This process he called the principle of natural selectionselection.
In Darwinism, the mechanism of natural selection is considered of major importance in the process of evolution. Popular formulations sometimes envisage a struggle for existence in which direct competition for mates or for various factors in the environment (e.g.
..... Click the link for more information. (the expression "survival of the fittest" was later coined by Herbert Spencer). In the same way, sexual selection (factors influencing the choice of mates among animals) also plays a part. In developing his theory that the origin and diversification of species results from gradual accumulation of individual modifications, Darwin was greatly influenced by Sir Charles Lyell's treatment of the doctrine of uniformitarianismuniformitarianism,
in geology, doctrine holding that changes in the earth's surface that occurred in past geologic time are referable to the same causes as changes now being produced upon the earth's surface.
..... Click the link for more information. . Darwin's evidence for evolution rested on the data of comparative anatomy, especially the study of homologous structures in different species and of rudimentary (vestigial) organs; of the recapitulation of past racial history in individual embryonic development; of geographical distribution, extensively documented by Wallace; of the immense variety in forms of plants and animals (to the degree that often one species is not distinct from another); and, to a lesser degree, of paleontology. As originally formulated, Darwinism did not distinguish between acquired characteristics, which are not transmissible by heredity, and genetic variations, which are inheritable. Modern knowledge of heredity—especially the concept of mutation, which provides an explanation of how variations may arise—has supplemented and modified the theory, but in its basic outline Darwinism is now universally accepted by scientists.
the materialist theory of the evolution (historical development) of the organic world on earth, based on the views of C. Darwin.
His observations during a round-the-world journey on the Beagle, together with research and generalization of the achievements of contemporary biologists and breeders, provided Darwin with the groundwork for his theory of evolution. Although he had begun to work out a theory of evolution in 1837, it was not until 1858 that Darwin, at a meeting of the Linnaean Society in London, first read a lecture that contained the basic precepts of the theory of natural selection. At the same meeting A. Wallace delivered a report expressing views that coincided with Darwin’s. Both lectures were published together in the journal of the Linnaean Society, but Wallace recognized that Darwin had developed the theory of evolution earlier, more profoundly, and more completely, and he entitled his basic work (issued in 1889) Darwinism, emphasizing thereby Darwin’s priority.
Darwin’s book. On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life, was published in 1859. Darwin demonstrated in this book that a transformation in a stock of domestic animals or cultivated plants takes place on the basis of minor changes in the traits of individual organisms. Man consciously selects organisms that possess the characteristics most valuable from an economic point of view, preserves them, and obtains offspring from them—in other words, he carries out artificial selection. Darwin proved that an analogous process can be found in nature. The hereditary changes that arise among animals and plants are subject to the effects of natural selection, so that in the struggle for existence those forms survive that are best adapted to given environmental conditions. Thus, Darwin explained by a materialist approach the expediency of organization in living things, in contrast to earlier attempts to create a theory of evolution on the assumption of the inherent capacity of organisms to change in response to external influences and to transmit those adaptive changes to their offspring. Thus, for the first time in the history of biology, he constructed a theory of evolution specifically guided by data obtained from economic practice. This was of great methodological significance, since it both permitted the idea of organic evolution to be substantiated clearly and convincingly for contemporaries and also allowed the validity of the theory itself to be tested. Darwin’s theory quickly won recognition despite fierce criticism, because the concept of historical development in nature explained observable facts better than the idea of the immutability of species; its rapid acceptance was due also to the advocacy of a number of biologists, including J. Hooker, A. Wallace, and T. Huxley in Great Britain; E. Haeckel, F. Müiller, and A. Weisman in Germany; A. Gray in the United States; and K. A. Timiriazev and M. A. Menzbir in Russia. K. Marx and F. Engels and later V. I. Lenin fully appreciated the significance of Darwinism and considered it the natural-historical basis of dialectical materialism. The materialist character of Darwinism made it unacceptable to conservative elements among biologists and to religious groups. Almost simultaneously with the triumph of the concept of the historical development of life, numerous anti-Darwinist theories of evolution began to arise, the authors of which suggested explanations of the motive forces of the process that were different from Darwin’s; these anti-Darwinist theories were often based upon idealistic or mechanistic world views; they included the theories of autogenesis, aristogenesis, bath-mogenesis. and orthogenesis.
Darwin’s theory proceeded from the existence of two basic types of variation—definite, defined as the adaptive reactions of organisms to the influence of external environmental factors, and indefinite, also arising under the influence of external factors but without any necessarily adaptive character. As a rule, definite changes disappear by the following generation in the absence of the factor that caused them. Indefinite changes, on the other hand, are transmitted from generation to generation independently of environmental conditions; Darwin therefore believed that it was precisely indefinite variation that provided the basic material for evolution. For a long time the idea of indefinite variation as the material for evolution remained the aspect of Darwinism most open to criticism. For example, the English engineer F. Jenkin believed that if individuals characterized by a given trait were crossbred with individuals not possessing that trait, the offspring obtained thereby would necessarily express that trait in only half the original degree. Consequently, in a series of generations new characteristics should disappear rather than take hold; evolution was therefore impossible. It was later demonstrated (Mendelian laws) that the inherited characteristics of organisms are not “diluted” in their transmission to subsequent generations. A number of anti-Darwinist theories, such as Lamarckism and neo-Lamarckism. attributed the predominant role in evolution to definite variation. According to these theories, adaptive changes are transmitted by inheritance and are the material for the evolutionary process. However, according to contemporary concepts, adaptive changes, or adaptive modifications, are not inherited. Only the capacity of organisms for the adaptive reactions to corresponding external factors is determined by heredity, as is evidenced by the constant reappearance of adaptive modifications with the restoration of previous conditions. In addition, numerous experiments have demonstrated that newly-appearing inherited changes in organisms are, as a rule, not adaptive. Adaptive modifications, on the other hand, are not new adaptations, but rather reactions developed in the course of the previous evolution of these organisms.
Only inherited variations (based, as established by contemporary biology, on mutations) and their combinations (the result of interbreeding) can serve as the material for evolution. New mutations are usually harmful—they disturb the adaptation already achieved. However, evolution does not reduce only to the sudden rise of new, successful, inherited characters. The interaction of organisms with the surrounding environment is expressed in the struggle for existence. According to Darwin, this phenomenon is caused by a shortage of vital resources (for example, food, light, shelter, and territory) for all individuals of a given species. In the struggle for existence among individuals, those which turn out to be poorly adapted to the given environmental conditions may suffer a decrease in fertility or die. The more closely related the organisms living in one territory, the more acute the competition among them and the greater the number that die. Those individuals survive that use different food, possess different means of defense, and so forth—in other words, those that acquire different characteristics. As a result, in a series of generations a divergence of traits occurs that leads ultimately to the splitting of the initial species into varieties, which in turn can become new species. Deviations that do not correspond to the environmental conditions are not preserved; individuals possessing such traits perish, but minor mutations are combined in the interbreeding of the individuals that pass selection. This leads to a change in the characteristics of the organism. Thus, because of the death of individuals possessing clearly nonadaptive deviations and the interbreeding of the survivors, the initially nonadaptive mutations are transformed in the process of selection into new adaptations. Since it is not individual traits that perish and survive in the struggle for existence, but rather the individuals bearing those traits, only a population (a group belonging to one species with individuals inhabiting the same territory constantly interbreeding among themselves) can actually evolve. Interbreeding that proceeds under natural selection leads not only to the change of mutations, but also to the gradual spread of the new adaptations to all individuals of the population. Because of the uninterrupted operation of selection in the process of evolution, there is an accumulation of new adaptive changes for the traits that are being selected. But all the parts of any single organism are most closely related to one another, and therefore correlated variation arises in the course of evolution. The gradual change in the structure of organisms corresponding to external environmental factors ultimately leads to the formation of new species. The concrete direction of evolution is determined on one hand by natural selection and on the other by the spectrum of indefinite inherited deviations among the organisms constituting the population that may be subject to that selection. Thus, inherited variation is the only material for evolution. Natural selection is the principal motive factor of evolution.
An important principle of Darwinism is the concept of the relative adaptability of organisms—that is, the idea that the adaptation of organisms to external environmental conditions, the expediency of their structure and functions, is imperfect. This relative character of adaptation also brings about evolution and compels organisms continually to improve in the process of selection. The recognition of organic expediency as an inherent characteristic of living organisms leads either to the complete rejection of evolution (organisms are ideally adapted to environmental conditions and are not subject to changes—the Creation theory) or to the postulation of an evolutionary process based on the inheritance of acquired characteristics and properties (the organism can react adequately and with expediency to changes in the environment and this reaction is strengthened among its descendants). However, there has not yet been convincing proof that such a process takes place.
Credit belongs to Darwin for the discovery of the motive forces of organic evolution. The subsequent development of biology has deepened and supplemented his ideas, which serve as the basis of contemporary Darwinism. The development of Darwinism has stimulated the progress of many branches of biology. In all biological disciplines the leading place is now occupied by the historical method of research, which permits the study of the concrete paths of evolution and penetration into the essence of biological phenomena. As a result of evolutionary treatment, the facts obtained by science in their turn promote further exploration of the problems of Darwinism. The works of Soviet biologists A. N. Severtsov and I. I. Shmal’gauzen. as well as a number of scientists abroad, such as G. de Beer, J. Huxley, T. Dob-zhansky, B. Rensch, G. Simpson, and others, have elucidated many of the laws of evolution (for example, autonomous development, adaptation genesis, the biogenetic law. macroevolution, microevolution. and phylembryogenesis).
Contemporary Darwinism is the most important theoretical basis for biology, agriculture, and medicine; only a consistent Darwinist approach makes possible the effective transformation of breeds of domestic animals and varieties of cultivated plants and the introduction of new. more productive strains of the microorganism-producers of antibiotics. Darwinism creates a basis for the concept of the biosphere as a complex evolving system; it will make possible in the future the management of the evolutionary process. Darwinism has in essence a no less important methodological significance, since the theory stands completely upon the positions of dialectical materialism, constantly providing material for the further development of the philosophical and methodological problems of contemporary natural science.
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