Evolution, Doctrine of

(biology), the body of knowledge of the historical development (evolution) of living nature. The doctrine of evolution encompasses the analysis of the development of adaptation, the evolution of ontogeny (or the individual development of organisms), the factors shaping evolution, and the concrete paths of the historical development (phylogeny) of individual groups of organisms and of the organic world as a whole. Based on the theory of evolution, it is also concerned with the origin of life and the origin of man. (SeeONTOGENY; PHYLOGENY; and ORIGIN OF LIFE.)

History. The earliest ideas about the development of life, as expounded in the works of Empedocles, Democritus, Lucretius, and other ancient Greek and Roman philosophers, were essentially brilliant conjectures unsupported by biological facts. In the 18th century, transformism, the doctrine of variability of animal and plant species, emerged in biology, in opposition to creation-ism, a doctrine based on the concept of divine creation and the immutability of species (seeTRANSFORMISM and CREATIONISM). The leading transformists of the second half of the 18th century and first half of the 19th—G. de Buffon and E. G. Saint-Hilaire in France, E. Darwin in England, J. W. van Goethe in Germany, and K. F. Rul’e in Russia—substantiated species variability mainly by two facts: the existence of transitional forms between similar species and the unity of the structural plan of the organisms of the major groups of animals and plants. However, the transformists did not examine factors responsible for the changes in species.

The first attempt to formulate an integral theory of evolution was made by the French naturalist J. B. de Lamarck, who set forth his ideas about the motive forces of evolution in Philosophie zoologique (1809). Lamarck believed that the transition from the lower forms of life to the higher forms— gradation—occurs as a result of the immanent and universal striving of organisms for perfection. He ascribed the diversity of species at every level of organization to the gradation-modifying influence of the environment. According to his first “law,” the use of organs leads to their progressive development, while disuse leads to their reduction. According to his second law, the results of use and disuse of organs when sufficiently prolonged become hereditarily fixed and are then transmitted from generation to generation independently of the environmental influences that initially produced them (seeLAMARCKISM and ACQUIRED CHARACTER). Lamarck’s laws were based on the erroneous assumption that nature strives for perfection and that favorably acquired characters are heritable.

The true factors responsible for evolution were disclosed by C. Darwin, who thereby formulated a scientifically based theory of evolution, set forth in the book The Origin of Species by Means of Natural Selection; or, the Preservation of Favored Races in the Struggle for Life (1859). The motive forces of evolution, according to Darwin (seeDARWINISM), are as follows: indefinite variation, the hereditarily caused diversity of organisms in every population of any species; the struggle for existence, in the course of which the less adapted organisms die or are prevented from reproducing; and natural selection, the survival of better adapted individuals, as a result of which useful hereditary changes accumulate and new adaptations arise. Lamarckism and Darwinism are diametrically opposite in their interpretation of evolution. The former explains evolution by adaptation, while the latter explains adaptation by evolution. Other ideas exist in addition to Lamarckism that reject the importance of selection as the motive force of evolution (seeAUTOGENESIS, MUTATIONISM, and NOMOGENESIS). Advances in biology soon confirmed the validity of Darwin’s theory. Therefore, Darwinism and the doctrine of evolution are often used synonymously in modern biology. Also similar in meaning is the term synthetic theory of evolution, which emphasizes the synthesis of the main tenets of Darwin’s theory and of genetics and certain generalizations on evolution from other branches of biology.

Modern doctrine of evolution. Advances in genetics helped clarify the mechanism of the origin of indefinite hereditary variation, which provides the material of evolution. This phenomenon is based on stable changes in inherited structures—mutations (see MUTATION). Mutation is not directed: newly occurring mutations do not fit the existing environmental conditions and generally disrupt already existing adaptations. Mutation acts as the principal material of evolution for organisms without a formed cell nucleus. Combinative variation, the combining of genes in the course of sexual reproduction, is very important for organisms whose cells do have a formed nucleus.

A population is the elementary unit of evolution (seePOPULATION). The comparative isolation of populations results in their reproductive isolation—the restricted interbreeding between individuals of different populations (seeISOLATION). Reproductive isolation guarantees the uniqueness of the gene pool —the genetic makeup of every population—and, consequently, the possibility of its evolving independently (seeGENE POOL). The biological mixture of individuals constituting a population determined by combinative and mutational variation is manifested in the struggle for existence. Some individuals die, while others survive and reproduce. As a result of natural selection, the new mutations combine with the genes of already selected individuals, the phenotype alters, and new adaptations arise. Hence, selection is the principal motive force of evolution and the factor responsible for new adaptations, the transformation of organisms, and speciation.

Selection may be manifested in different forms: stabilizing selection guarantees the preservation of already formed adaptations under unchanged environmental conditions; directional selection leads to the formation of new adaptations; and disruptive selection gives rise to polymorphism when considerable changes occur in the habitat of a population. In the modern doctrine of evolution, the conception of the factors of evolution was enhanced by the identification of the population as the elementary unit of evolution, by the theory of isolation, and by the development of the theory of natural selection. Isolation as a factor responsible for increasing the diversity of living forms underlies modern thinking on speciation and species structure (seeSPECIATION). Allopatric speciation, caused by the distribution of a species and by the geographic isolation of distant populations, has been studied in greatest detail thus far. Less attention has been focused on sympatric speciation, caused by ecological, chronological, or ethological (behavioral) isolation (seeALLOPATRIC DISTRIBUTION OF ORGANISMS and SYMPATRIC DISTRIBUTION OF ORGANISMS).

The evolutionary processes that occur within a species and culminate in speciation are often combined under the general term “microevolution” (seeMICROEVOLUTION). Macroevolution is the historical development of groups of organisms (taxa) of categories above the level of species. The evolution of taxa higher than species is the result of speciation originating under the influence of natural selection. However, the use of different time scales (the evolution of major taxa encompasses many stages of speciation) and research methods (use of data from paleontology, comparative morphology, embryology, and other sciences) helps to reveal patterns that are not detectable in the study of microevolution. The most important objectives of macroevolution are to analyze the relationship between individual development and the historical development of organisms and to determine the regularities of phylogeny and the main directions of the evolutionary process.

In 1866 the German naturalist E. Haeckel formulated the biogenetic law, according to which the stages of phylogeny of a taxonomic group are recapitulated in ontogeny (seeBIOGENETIC LAW). Mutations are reflected in the phenotype of the adult organism if they alter the processes of its ontogeny. Therefore, natural selection among adult individuals leads to the evolution of the processes of ontogeny—the interrelations between developing organs, which I. I. Shmal’gauzen called ontogenetic correlations. The reorganization of the system of ontogenetic correlations caused by directional selection gives rise to phylembryogeny, by means of which new traits of organisms are formed in the course of phylogeny. If the changes occur in the organ’s final stage of development, the organs of the ancestors continue to evolve further (anaboly). Deviations in ontogeny may also occur in the intermediate stages, resulting in the restructuring of organs (deviation). Changes in the formation and development of the early rudiments may give rise to organs not present in the ancestors (archallaxis). However, the evolution of ontogenetic correlations under the influence of stabilizing selection results in the preservation of only those correlations that most reliably safeguard the processes of ontogeny. These correlations are the recapitulations —duplications of the phylogenetic states of the ancestors in the ontogeny of the offspring—and they confirm the biogenetic law.

The direction of phylogeny of every taxonomic group is determined by its actual relation to the environment in which the taxon and its organization are evolving. The divergence of traits that arises among two or more taxa with a common ancestor is due to differences in environmental factors. It begins at the population level, leads to an increase in the number of species, and continues at taxonomic levels above the level of species. The taxonomic diversity of living organisms is caused by divergent evolution. Parallel evolution, which is less common, occurs when primary divergent taxa remain under similar environmental conditions and develop similar adaptations based on a similar organization inherited from the common ancestor. Convergence occurs when unrelated taxa adapt to identical conditions. Biological progress can be achieved by a general rise in the level of organization responsible for adapting organisms to environmental conditions that are broader and more varied than the conditions under which their ancestors lived. Such changes, aromorphoses, rarely occur and must give way to allomorphoses—divergence and adaptation to more specific conditions in the new habitat (seeAROMORPHOSIS and ALLOMORPHOSIS).

The formation of narrow adaptations in the phylogeny of a group results in specialization. Shmal’gauzen distinguished four main types of specialization—telomorphosis, hypomorphosis, hy-permorphosis, and katamorphosis—which, although differing in the nature of the adaptations, all result in a slowing of the rate of evolution (seeEVOLUTION, RATE OF) and a decrease in evolutionary flexibility as a result of the loss of multiple functions by the organs of specialized animals (seeTELOMORPHOSIS; HYPOMORPHOSIS; HYPERMORPHOSIS; and KATAMORPHOSIS). If the environmental conditions remain stable, the specialized species can exist indefinitely. This explains the occurrence of “living fossils,” such as the many mollusk and brachiopod species that have existed from the Cambrian to the present day. If the living conditions change abruptly, the specialized species die out, while the more adaptable ones manage to adapt to the changes.

The doctrine of evolution and especially its theoretical core, the theory of evolution, serve as an important scientific foundation of dialetical materialism and a basis for modern biological methods.