Mutation Theory

a theory of variation and evolution advanced at the beginning of the 20th century by H. De Vries.

According to the mutation theory, of the two categories of variation, continuous and discontinuous (discrete), only the latter types of variation are inherited; De Vries called discontinuous variation mutation. He believed that mutations can be progressive or regressive. Progressive mutations involve the appearance of new hereditary properties. This process is equivalent to the origin of new elementary species. Regressive mutations, which involve the loss of existing properties, signify the origin of varieties. New elementary species, or jordanons (micro-species), arise suddenly by progressive mutations without the appearance of transitional types and usually become hereditarily fixed. The appearance of large numbers of mutations is confined to very rare mutation periods that alternate with long resting periods in the life of each species. De Vries’ conclusions were based mainly on his observations of the plant Oenothera lamarkiana. Although at one time these conclusions greatly accelerated mutation research, by the 1920’s the development of genetics refuted all the main principles of the mutation theory.

A similar system of ideas concerning variation and evolution was elaborated in 1899 by S. I. Korzhinskii, who described many proven cases of the sudden appearance of isolated, discrete hereditary changes in plants. These changes were not related to previous crossbreedings or to the influence of environmental conditions. In the elaboration of his theory of evolution based on heterogenesis, Korzhinskii called such changes heterogeneous.

Korzhinskii’s heterogeneous variations are closer in meaning to the modern term “mutation” than are De Vries’ mutations. Both the acceptance of discontinuous variation as having fundamental significance in evolution and the denial of the role of natural selection in the theories of Korzhinskii and De Vries were due to the failure at that time to resolve the contradiction in Darwin’s evolutionary theory between the importance of minor variations and the absorption of these variations through crossbreeding. This contradiction was resolved by the formulation of the modern view of heredity and by S. S. Chetverikov, who in 1926 combined this modern view with the concepts of evolutionary theory.