J Hered.  70: 301–308. (1979)
Destabilizing selection as a factor in domestication.
Dmitry K. Belyaev

Darwinian, Stabilizing, and Destabilizing Selection

What is the nature and the essence of the selection that serves as the driving force of domestication? What is the form, or rather the effect, of this selection? The modern literature on evolutionary genetics knows many forms of selection. After Darwin, who distinguished natural and artificial selection by a formal criterion, a series of authors have described other forms of selection (conserving, disruptive, etc.). Schmalhausen14  distinguished two main modes of selection: promoting selection (the main force of evolution as argued by Darwin); and stabilizing selection, the theory of which was formulated by Schmalhausen  himself.

Darwinian selection is based mainly on mutations of minor effect that lead to a very slow shift of the average adaptedness of populations and species to the conditions of new ecological niches within a given environment. As Darwin wrote, these selection processes do not generate any new variation, but use only what already exists. Modern population genetics and the dominant theory of evolution are based on this type of selection. Stabilizing selection operates under conditions of a relatively stable environment to which the species is well adapted. It provides for stabilization of ontogeny to develop the optimum phenotype for the given environment. Stabilizing selection displays its effect through the elimination of mutations disturbing the normal ontogeny and phenotype. It maintains variation within the optimum limits developed by evolution, but does not create new variations. However, all modes of selection result in discarding the unfit. This was well understood by Schmalhausen, the second (posthumous) edition of whose "Factors of Evolution" ended as follows: "In conclusion I want to point out that the stabilizing selection in its concrete manifestation is not a special form of selection. It would be more correct to speak of promoting and stabilizing effects (italics added) of the general process of natural selection."

Fig. 4—Foxes with brown piebaldness

However, selection for tame behavior seems to result in breaking up previously integrated ontogenetic systems and thus leads to multiple phenotypic effects that seem genetically unrelated to the selected character, namely tame behavior. In a genetic and biochemical sense, what may be selected for are changes in the regulation of genes—that is, in the timing and the amount of gene expression rather than changes in individual structural genes. Selection having such an effect is called by me destabilizing selection2-4. The selection becomes destabilizing when it affects, directly or indirectly, the systems of neuroendocrine control of ontogenesis. This seems always to be the case when some new stressful factors appear in the environment, or when stresses usual for the species increase in strength. One may think therefore that stress is one of the important factors accelerating the evolution of life, especially at the highest level of organization where neuro-hormonal controls on ontogeny are most effective. Destabilizing selection2-4 could break up normal patterns of gene activation and inhibition and result in a great increase in the range and rate of hereditary variation, which again is subject to specific selective forces. The process of domestication in all animal species seems to have resulted in the same kinds of homologous variations as a result of selection for the single important characteristic of tame behavior. Foxes are only the latest animals to be added to this list of domesticated species. The destabilizing effects of domestication can also lead to many undesirable characteristics, but further selection tends to eliminate or modify these characters. The biological principles of selection can also. of course, apply to human beings. During our evolution we have not escaped from the stressful conditions under which destabilizing selection may operate. The objective dialectics of life consist in a contradictory unity of good and evil and we must see clearly this simple yet indisputable truth when attempting to estimate the prospects of evolution of life on our beautiful planet.

  1. BELYAEV, D.K. Domestication of animals. Science J. 1:47-52. 1969.
  2. ———— Biological aspects of domestication of animals. Proc. of the All-Union Conf. on Genetics & Breeding of Agricultural Animals. Alma-Ata, p. 30-44, (in Russian) 1970.
  3. ———— Some questions of stabilizing and destabilizing selection. In History and Theory of the Evolutionary Doctrine (in Russian). Leningrad, p. 76-84. 1974
  1. SCHMALHAUSEN, I.I. Factors of Evolution. Nauka Publishing House, Moscow (in Russian). 1968.