Fluctuations

FLUCTUATIONS, a term proposed by de Vries, designating changes in the phenotype of an organism that do not depend either on changes in its hereditary determinants (genes) or on their combination. A synonym for this term is "modification," proposed considerably earlier by Nägeli and still used on a par with the term "fluctuation." De Vries contrasted fluctuations with mutations (see), which represent changes in genes inherited according to Mendel's laws. Fluctuations are the result of various external influences on the body of an animal or plant and are encountered even in genotypically completely homogeneous material, in so-called pure lines (see). Fluctuations are observed both under natural conditions and can be artificially induced by changing the conditions under which the development of the organism takes place. Fluctuations can be caused by changes in nutrition, temperature, illumination, pressure, etc. The magnitude of the change can in no way serve as a criterion for distinguishing between mutations and fluctuations. As an example of a strong, non-heritable change—which is consequently a fluctuation—one can cite the case of the brine shrimp Artemia salina. Schmankewitsch, and later Gaievskaya, demonstrated that changing the salt concentration in a body of water can cause this crustacean to develop characteristics typical of the genus Branchipus. Thus, the sole criterion for distinguishing between a mutation and a fluctuation is their heritability. Fluctuations of quantitative traits (such as height, weight, volume, etc.) are especially widespread. Fluctuations of quantitative traits can be presented in the form of a variation series or graphically depicted as a variation curve (see Variation statistics). These variation series (or curves) are usually binomial, meaning that the distribution of individuals within them follows the coefficients of a binomial raised to a certain power. The latter proposition is often called Quetelet's law. Quetelet's law is the fundamental law of individual or fluctuating variability. The cause of such a binomial distribution of variants in a variation series is the multiplicity of influences acting on the organism. During its development, an organism is affected by a large number of external factors, some of which increase the dimensions of a trait, while others, conversely, decrease them. Most organisms are acted upon by factors both increasing and decreasing the dimensions of a trait, making the middle class of the variation series the most frequent one. A smaller number of individuals are subjected to external influences acting in only one direction. The result of this is the generally observed gradual decrease in frequencies in the variation series as the distance from its arithmetic mean increases. Since fluctuations are not associated with changes in genes and consequently are not heritable, they cannot serve as material for either natural or artificial selection (breeding). This has been most convincingly proven by Johannsen's classical experiments in his work on pure lines in beans. In practical breeding work, it is important to be able to distinguish fluctuations from mutations, which serve as material for both natural and artificial selection.