The Synthesis Of S. S. Chetverikov

The development of early twentieth century evolutionary biology was not restricted to a handful of scientists in Great Britain and the United States; it was a truly international scientific effort.  One of the major figures during this period was Sergei S. Chetverikov (1880-1959), but his contributions were unappreciated until after his death.

Chetverikov's appreciation of the nature of the evolutionary process was extraordinary given that he did most of his work when experimental and theoretical population genetics were in their infancy.  The summary of his seminal 1926 paper is still a succinct statement of the basic principles of population genetics, which is why it is included here.  Chetverikov, however, was more than just a conceptual theoretician; he was an experimentalist and teacher as well.  His work with Drosophila was one of the first demonstrations of the widespread genetic variation in natural populations, and his influence on N. Timofeev-Ressovsky, B. L. Astaurov, N. P. Dubinin, and, to a lesser extent, Theodosious Dobzhansky established the foundation for a strong program in evolutionary genetics.

Tragically, the life of the Russian branch of evolutionary biology was short.  The "Chetverikov school" was destroyed during the same period of Stalinist purges that disrupted so much of Russian society.  Specifically, a charlatan by the name of T. D. Lysenko organized an attack on all legitimate genetics research in the Soviet Union.  Many of Chetverikov's students were affected by this movement, but the most visible victim of the Lysenkoist purges was the brilliant agricultural geneticist, N. I. Vavilov, because he stood in the direct path of Lysenko's rise to power.  Chetverikov himself lost his academic position in the early, general purges.  He was briefly a consultant to a zoo in the Ural mountains, then a mathematics instructor at the equivalent of a junior college, and, finally, before genetics research was outlawed altogether in 1948, he was able to do some research on selection in a species of silkworm.  He died impoverished and blind just as the Soviet government was realizing that it had to redevelop a program of legitimate research in all areas of Genetics.

A full English translation of the paper can be found in Chetverikov (1961).  Related historical details can be obtained from the papers by  Adams (1968, 1980), Darlington (1977), Berg (1979), Paul (1983), and the books by Medvedev (1969), and Joravsky (1970).
 


AN EDITED VERSION OF THE CLOSING COMMENTS FROM:

Chetverikov, S. S.  1926.  On certain aspects of the evolutionary process from the standpoint of modern genetics.  Translation by M. Barker and I. M. Lerner.
     1961.  Proc. Amer. Phil. Soc.  105:167-195.

1. The process of mutation is the same in natural, laboratory, and domesticated populations.

2. Most mutations are deleterious, but there are undoubtedly some that do not reduce viability.

3. A species population, in the conditions of random mating (Chetverikov used the term "free crossing") is a stable aggregate, within which the apparatus which
    stabilizes the frequencies of the component allelic pairs is an intrinsic feature of random mating.

4. Each newly arising mutation is absorbed by the species in the heterozygous state.

5. "From year to year, from generation to generation, more and more new mutations arise, either similar to the preceding ones, or completely new.  They are
     constantly absorbed into the basic species, which continually preserves its external homogeneity.  This heterozygosity saturates the species in all directions,
     recombining and spreading in accordance with the laws of chance (in so far as the various genes are not linked with one another), and gradually "contaminates"
     the majority of the individuals of the species."

6. With a sufficiently great number of mutations, almost all individuals carry various heterozygous and recessive mutations.

7. As mutations accumulate in a population, the probability of a mutant appearing in the homozygous condition increases.  The species begins to show more and
    more heritable deviations, and it begins to show instability in its characters.

8. "The most favorable conditions for the appearance of genotypic variability are provided when a numerous species breaks up into a series of small isolated
      colonies".

9. "Isolation, together with continuously arising genotypic variability, is a basic factor in intra-specific (and hence, interspecific) differentiation.  Most commonly, this
     isolation is spatial but sometimes it may be temporal, and, perhaps, environmental (ecological isolation)".
 

10. "Natural selection is in essence an antagonist of random mating (S. S. C. used "freecrossing")...It is a dynamic principle".

11. "Norton's table shows that every evolutionary process evoked by selection, regardless of whether the genes are dominant or recessive, always proceeds to the
       end, to the complete replacement for the less adapted form by the more adapted one.  It also demonstrates that selection takes up and fixes every, even the
       most insignificant, improvement in the organism".

12. Adaptive evolution without isolation always leads to the complete transformation of the species, but never can lead to the subdivision of the species into two
      parts, speciation.

13. "The cessation of the action of selection leads to the formation of a polymorphic species".

14. Selection, like random mating, promotes the accumulation of recessive (and less viable) genes in the heterozygous condition in the population.

15. Research has shown that most mutations are recessive, this implies that species under natural conditions will accumulate recessive alleles due to the effects of
      random mating and selection.

16. The effects of random mating and selection are different.  Random mating leads to the accumulation of mutations and the differentiation of the population, while
      selection, in eliminating harmful mutations, decreases variability and leads to monomorphism of the species.

17. "We have no basis for denying the possibility of nonadaptive evolution.  On the contrary, in many cases one must assume that the existing adaptive differences
        between closely related forms were not the cause of their divergence, but, on the contrary, that the particular nature of these adaptive characteristics was a
        consequence of a still earlier individualization of these forms.  The more ancient such separation, the more adaptive characteristics will distinguish each form
        from others".

18. "The concept of the multiple action of genes (pleiotropy) advanced by Morgan is extremely important for an understanding of the action of selection.  This leads
        to the concept of the genotypic milieu, as a complex of genes, internally and hereditarily influencing the manifestation of every gene in its character.  Every
        individual is indivisible not only in so far as its soma is concerned, but also in the manifestation of every one of its genes".

19. "The concept of pleiotropic action of genes clarifies a series of difficult and involved questions of genetics:  enhancers, weakeners, modifiers of quantitative
       characters".

20. "Selection, in selecting not only the gene determining the selected character, but the whole genotype (genotypic milieu) leads to the reinforcement of the selected
       character and in this sense actively participates in the evolutionary process".

21. "The concept of the pleiotropic action of genes yields a new theoretical basis for the phenomenon of correlated variability and for the genotypical correlation of
       characters".