Stabilizing Selection

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Stabilizing Selection

 

(also negative selection), a form of natural selection that is responsible for the preservation of the adaptive characteristics of organisms under constant environmental conditions. Stabilizing selection operates by means of the removal, or elimination, of individuals who deviate from the established norm. Therefore, under the influence of stabilizing selection, a population remains unchanged for a given characteristic, despite the continuous process of mutagenesis.

Stabilizing selection ensures the preservation of persistent and bradytelic forms, as well as the preservation during phylogeny of old characteristics that have not lost their adaptive significance. For example, the structure of the thyroid gland’s hormone —thyroxine—has remained unchanged throughout the evolution of vertebrate animals. According to I. I. Shmal’gauzen, who developed the concept of stabilizing selection, there is an increase in the genetic diversity of a population during stabilizing selection. Hence, recessive alleles accumulate when the phenotype is preserved unchanged, and, as a result, the gene pool of the population is enriched. Thus, a ready reserve of hereditary variation is formed, which constitutes the latent genotypic diversity of a population. The reserve becomes the material for evolution when there are sharp changes in the environment and evolutionary (positive) selection, which is the alternative to stabilizing selection, is put into action. Evolutionary and stabilizing selection always coexist in nature, and for any given period of a population’s evolution it is only possible to speak of the predominance of one of these forms.

An important result of stabilizing selection is the perfection of ontogenetic processes. Stabilizing selection accumulates hereditary changes that cause the rapid and reliable development of the constant characteristics of an adult organism that have been preserved. For this reason, both Shmal’gauzen and the British biologist C. Waddington regarded the evolutionary origin of adaptive modifications to be the result of stabilizing selection. If a population adapts simultaneously to different environmental conditions, several channels of ontogeny are formed on the basis of a given genotype. The channels, which are balanced complexes of mor-phogenetic processes, cause the development of a phenotype that is adapted to certain conditions.

In light of the effects of stabilizing selection, Waddington and the American biologist T. Dobzhansky have distinguished two subforms of stabilizing selection: normalizing selection, which preserves formed adaptations, and channelizing selection, under whose influence ontogeny is perfected.

REFERENCES

Shmal’gauzen, 1.1. Faktory evoliutsii, 2nd ed. Moscow, 1968.
Shmal’gauzen, 1.1. Problemy darvinizma, 2nd ed. Leningrad, 1969.
Dobzhansky, T. Genetics of the Evolutionary Process. New York-London, 1970.

A. S. SEVERTSOV

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