modification(redirected from Moded)
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in biology, nonhereditary changes in the visible characters (phenotype) of an organism; these changes occur under the influence of altered environmental conditions.
Modifying factors in the environment (temperature, illumination, diet) act on the organism during sensitive periods of its development, precipitating changes in the organism’s internal makeup (for example, changes in hormone level) and altering the course of ontogeny. Modificational variation consists in changes within the limits of the genotypically conditioned norm of reaction. Depending on the temperature of its environment, the Himalayan rabbit may change from black in cold weather to rusty brown in mild temperatures to pure white in high temperatures. However, the coloration of the parent rabbits is not inherited by their young; only the capacity to change color according to temperature change is inherited.
In nature, modifications are, as a rule, adaptive reactions of organisms to environmental factors. Thus, in lake Sagittaria sagittifolia, the shape of the leaves depends on where they grow: the above-water leaves are arrow-shaped, the leaves that float are heart-shaped with stomata on the upper side, and the underwater leaves are ribbon-like. Nonadaptive modifications often consist of all kinds of developmental disturbances, the extreme expression of which includes deformities and morphological and physiological malfunctions. These modifications occur, as a rule, in response to strong external stimuli, to which individuals of a given species are rarely exposed under normal conditions.
In contrast to mutations, modifications are not transmitted by heredity. Modifications develop in individuals of a given generation only when the conditions under which these modifications normally arise are present. However, in unicellular and occasionally multicellular organisms, prolonged modifications are found when changes in visible characters that have arisen as a result of environmental factors are retained for several generations, even after the disappearance of the inducing factors. This type of modification is apparently caused by changes in relatively stable self-reproducing cytoplasmic structures. Since they are nonhereditary, modifications have no direct evolutionary significance. If, however, a phenotype formed as a result of modification has a high adaptive value, the phenotype may become fixed in evolution only through the selection of those mutants that retain a given modification, but discard other modifications of a given character.
REFERENCESLobashev, M. E. Genetika, 2nd ed. Leningrad, 1967.
Shmal’gauzen, I. I. Faktory evoliutsii. Moscow, 1968.
N. V. TIMOFEEV-RESOVSKII, V. I. IVANOV and V. A. MGLINETS