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in biology, the limitation or disruption of free interbreeding between individuals and of mixing (panmixia) of different forms of organisms; one of the elemental factors of evolution.
Using island fauna and flora as an example, C. Darwin demonstrated the role of isolation in the emergence, expansion, and accentuation of differences between closely related forms of living organisms. If part (most often a peripheral part) of an original population is isolated by geographic obstacles, that part may in time be transformed into an independent species. This geographic (allopatric) means of species formation is, in the opinion of many biologists, the only, or, in any case, the main path of speciation. On the macroevolutionary level, isolation is a function of the inability of different species to interbreed, that is, it has a predominantly reproductive character. On the microevolu-tionary level (that is, on the intraspecific level), there are two principal types of isolation: territorial-mechanical isolation, which includes all cases of the emergence of barriers between different parts of a population or between different populations (for example, water to terrestrial organisms and dry land to aquatic organisms, or mountains to lowland species and lowlands to mountain species), and biological isolation, which is divided into three subgroups: (1) ecological isolation, in which individuals of two or more biotypes rarely or never meet during the reproductive period; (2) morphophysiological isolation, in which copulation is difficult or impossible for morphological or ethological (behavioral) reasons; and (3) strictly genetic isolation, caused by the defectiveness (decreased vitality or fertility or complete sterility) of the hybrids obtained from certain inter-breedings. All forms of isolation may exert various pressures on populations, since any form of isolation may be expressed quantitatively in varying degree. Territorial-mechanical isolation (or geographic, for large territories) leads to allopatric form development; given a sufficiently prolonged period of effect, it leads to the appearance of certain forms of biological isolation. Instances of the emergence of biological isolation may lead to sympatric development.
REFERENCESDarwin, C. Proiskhozhdenie vidov putem estestvennogo otbora. So-chineniia, vol. 3. Moscow-Leningrad, 1939.
Geptner, V.G. Obshchaia zoogeografiia. Moscow, 1936.
Ehrlich, P., and R. Holm. Protsess eveoliutsii. Moscow, 1966. (Translated from English.)
Shmal’gauzen, I.I. Faktory evoliutsii, 2nd ed. Moscow, 1968.
Timofeev-Resovskii, N. V., N.N. Vorontsov, and A.V. Iablokov. Kratkii ocherk teorii evoliutsii. Moscow, 1969.
Shmal’gauzen, I.I. Problemy darvinizma, 2nd ed. Leningrad, 1969.
V. G. GEPTNER and N. N. TIMOFEEV-RESOVSKII
the intonational and semantic separation of part of a sentence (together with dependent words), making it syntactically independent.
Intonational separation is accomplished by raising the voice before the part to be isolated and by using pauses and phrase stress. Word order is frequently changed. The semantic and stylistic function of isolation consists in the more precise expression of an idea or the further description of a person or object.
Isolation often lends expressive color to a sentence. In writing, the isolated parts of a sentence are usually set off by commas or, more rarely, by dashes. For example, “Right opposite the military post, on the far shore, it was deserted” (L. N. Tolstoy).