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

References in periodicals archive ?
Among them, 13 significant selected unigenes were related to disease resistance and ten of them subjected to purifying selection (Supplementary Table 1).
According to M2 model, 65% sites were found under purifying selection, 28% under neutral evolution and only 6% were protected by positive selection.
It is well known (Kryazhimskiy and Plotkin, 2008; Anisimova and Liberies, 2012; Mugal et al., 2014) that violating this assumption may have a high associated risk of identifying false positives in analyses of positive selection (e.g., when alignments of genes under purifying selection include rare nonsynonymous mutations that have not yet been eliminated from the population by selection), or of false negatives (e.g., when stochastic variation in the number of synonymous polymorphisms within a single population masks the underlying effects of positive selection for high rates of nonsynonymous substitution).
A dN/dS ratio of 1.0 is an indicator of neutral evolution whereas dN/dS > 1.0 is an indicator of diversifying selection and dN/dS < 1.0 is an indicator of purifying selection [20,21].
The dosage effect model implies that paralogs are subject to purifying selection from the onset of evolution after the gene duplication [3, 7] whereas the DDC model assumes "constructive neutral evolution" [14] whereby the paralogs are maintained due to the partial, differential degeneration of their functions resulting in functional complementarities [2, 6, 35].
The rationale is as follows: a gene, such as a histone, which is highly constrained by purifying selection, will diverge only very slowly between species and will also exhibit very little polymorphism, whereas an unconstrained gene, say a pseudogene, will diverge rapidly and exhibit high levels of polymorphism.
The results of the positive selection analyses indicated that Tas2r10 and Tas2r67 had positively selected sites and other intact genes in raccoon dogs were under purifying selection. Overall, the phylogenetic and evolutionary relationship of Tas2r genes in raccoon dogs was first studied in this study.
Compared with rice ARF genes, 18 pairs of orthologs originated from positive selection (Ka/Ks ratio was larger than 1), while 6 orthologs showed purifying selection (Ka/Ks ratio was less than 1) (Table 2).
But comparative analysis of plant genomes also point to ancient genes, highly conserved and under strong purifying selection. Those are most often involved in the metabolism of molecules essential for plant development and with signaling function.
The numbers of synonymous (dS) and nonsynonymous substitutions (dN) per synonymous and non-synonymous sites, respectively, were used to calculate the test statistic (dN-dS) along with the probability of rejecting the null hypothesis that the codons have evolved through neutral selection (dN = dS), against the alternative hypothesis of evolution of the codon through positive selection (test of positive selection; dN>dS) or through purifying selection (test of purifying selection; dN<dS).
The DK-A and DK-B groups differed significantly in selection pressure: DK-A, [d.sub.N]/[d.sub.S] ratio = 0.195 (95% CI 0.105-0.328); and DK-B, [d.sub.N]/ [d.sub.S] ratio = 0.033 (95% CI 0.015 0.062), which indicates stronger purifying selection on the DK-B group.
In one, a single allelic state is identified as the neutral state and all other allelic states are deleterious (this models purifying selection).