S. S. Chetverikov

I have been reading Chetverikov's 1926 essay "On Certain Aspects of the Evolutionary Process from the Standpoint of Modern Genetics." I am posting my notes on the essay here, since I think his work would be well worth disseminating, as part of a broader study of the contribution of Soviet geneticists of the 1920s to the great evolutionary synthesis. I will comment more on his work in future posts. These are mainly excerpts from what I take to be the most important sections.

Sergei Sergeevich Chetverikov (1880-1959) was one of the first theoreticians of the evolutionary synthesis, a scientific movement that sought to place evolutionary theory firmly within the framework of modern genetic theory. He was apparently involved in anti-government activity as a student in Moscow around the time of the revolution of 1905. He was arrested at one point during this period. He was arrested and banished from Moscow in 1929, during the early period of Stalinist repression, perhaps for being involved in an exclusive genetics discussion group, being opposed to the theory of the inheritance of acquired characteristics, or after being denounced to the political police by one of his students. His work on genetics ended conclusively in 1948 with the rise of Lysenkoism. Chetverikov had a profound impact on other thinkers of the synthesis, particularly Ernst Mayr. In addition to theoretical work, he conducted important studies of the genetics of Drosophila melanogaster, the fruit fly, building on the research of Morgan. He used an inbreeding technique to reveal recessive alleles.

The basic question of the evolutionary synthesis, as formulated by Chetverikov: “How can one link evolution with genetics, and bring our current genetic notions and concepts within the range of those ideas which encompass this basic biological problem? Would it be possible to approach the question of variability, the struggle for existence, selection, in other words, Darwinism, starting not from those completely amorphous, indistinct, indefinite opinions on heredity which existed at the time of Darwin and his immediate followers, but from the firm laws of genetics?”

1. The Origins of Mutations in Nature

Problem of the relative absence of mutations in nature, compared to the number of mutations observed during the course of laboratory work.

Genetics is the basis for speciation and evolution. It is not simply a purifying mechanism, in which some basic species substance is conserved throughout all time. “We have already seen in several examples mentioned above that mutational variability touches upon various characters greatly different in significance. Alongside of the least salient traits, such as the color of the body, such important characters of Drosophila are changed as venation, wing structure, etc., which are fundamental in the modern systematics of insects for distinguishing the higher systematic categories. Consequently, it is necessary to acknowledge as completely erroneous the idea which some express, that mutations deal in only a superficial way with species traits, being characteristic of differences between varieties, while, apart from these small aberrations, there is a ‘basic substance’ of organisms which is not subject to mutational changes. Because of this, it is argued, the process of evolution, the process of the transformation of whole organisms into others, could not be achieved by means of mutations. Speaking figuratively, it is claimed that with all mutations, a fly always remains a fly, and a rat a rat, and never does the latter produce deviation in the direction of a rabbit or a dog.

“But here two concepts are confused: the diversity of characters subject to genotypic variability, and the scope of variability, i.e., the amplitude of deviation. Actually, we see that absolutely all parts of the organism are subject to genotypic variation. But, although genotypic variability is discontinuous, its leaps, naturally, cannot be infinitely large, so that the amplitude of aberration is limited, and the limit is determined by the structure of the genes themselves. Abrupt and profound changes of the organism are possible only be means of a prolonged accumulation of mutational changes, of long-termed stratification of one deviation upon another.”

2. Mutations under conditions of free crossing

Formulation of the biological species concept. “The definition of species, as an aggregate of individuals constituting a single freely intercrossing complex…” That is, a species is a unity of difference. Morphological difference by itself does not imply the existence of two species. “And from our genetic experiments on the most diverse animal and plant organisms, we now known that it is possible to create two groups of such organisms, which will differ from each other by a perfectly concrete complex (theoretically speaking, as large as one wishes) of morphological characteristics, which are not connected by intermediate forms; that is, having a so-called morphological hiatus, but at the same time belonging genetically to the same species.” Incidentally, this highlights the difficulties associated with determining species structure when looking at fossil material.

Variation is essential to the species concept, and is necessary for the operation of natural selection. But how is variation maintained? Objections were raised to Darwin’s theory from an engineer, Professor Jenkins, that the free crossing of individuals (under the assumption of blended inheritance) would quickly dissolve any new deviation. Led Darwin to move away from hard inheritance in explaining the persistence of variation, to Lamarkian theories. A.R. Wallace refers to “the swamping affects of intercrossing.” To truly understand variation and the maintenance of variation, we need genetics.

Hardy’s law of equilibrium under free crossing. “the relative frequencies of homozygous (dominant as well as recessive) and heterozygous individuals, under conditions of free crossing and in the absence of any kind of selection, remain constant, provided that the product of the frequencies of homozygous individuals (dominant and recessive) is equal to the square of half of the frequency of heterozygous forms.” pr = q². “Since for any value of p and r a value of 2q may be found to satisfy the equation…a freely crossing population may be in a state of equilibrium with any proportions of homozygous dominant and recessive forms.”

Law of stabilizing crossing (Pearson’s law): “under conditions of free crossing with any initial ratio of frequencies of homozygous and heterozygous parental forms, a state of equilibrium will be established in the population as a consequence f the very first generation of free crossing. Thus, should a state of equilibrium in a freely crossing population be disturbed from without, as a result of the very first subsequent crossing, which we will call stabilizing crossing, a new state of equilibrium is established within the population, at which the given population will remain until some new external force removes it from this state.” Among these external forces, we may include selection and mutation.

These laws imply that mutations may appear and then subsist in a population in heterozygous form, only rarely being expressed phenotypically. “A species, like a sponge, soaks up heterozygous mutations, while remaining from first to last externally (phenotypically) homogeneous.” “The older the species, the more mutations are accumulated within it, the more frequently is one or another of them disclosed in the homozygous state, and the more the species becomes externally genetically variable. Generally speaking, all other conditions being equal, genotypic variability of a species increases proportionally to its age.”

The smaller the population size, the greater the probability that absorbed mutations will express themselves phenotypically. Basis for a theory of speciation based on isolation, which foreshadows Mayr’s work. “If we imagine that the total number of individuals of a given species N, is subdivided into a series of isolated colonies, then the frequency of origin of new mutations within the limits of the entire species will not suffer, but the probability of reappearance of each such mutation will be once more considerably increased, depending on the reduced size (n) of the colony, within which it originally arose. Thus, we approach a more profound understanding of the enormous role which the factor of isolation plays in the origin of species variability…A species, as we have tried to show above, represents limitless diversity of genotypic combinations, and each isolation creates in it at once the exceptionally favorable conditions for the manifestation of heritable variations...And so, isolation, under the conditions of a process of continuous accumulation of mutations becomes, by itself, a cause of intraspecific (and consequently, interspecific) differentiation. Of all the factors contributing to the break-up of a species into separate non-interbreeding colonies, it is necessary, naturally, to put spatial, geographical isolation in the first place as the most powerful and common factor of intraspecific differentiation.” Also, isolation in time (different mating cycles), or ecological isolation. Isolation at the margin of a population, where the struggle for existence is more intense.

3. Natural selection

Free crossing vs. natural selection: “In the foregoing analysis of free crossing, we tried to establish its role as a factor stabilizing a given population. In its very essence it is a conservative factor, preserving the genotypic composition of the species in the condition in which it is found at a given moment. Natural selection (and, in general, selection in any form) is, in this connection, its direct antagonist. If free crossing stabilizes the population, then selection, on the contrary, all the time displaces the equilibrium state, and, if in this sense we may call free crossing a conservative principle, then selection, undoubtedly, is the dynamic principle, leading ceaselessly to modification of the species.”

Intensity of selection. Difference in the mechanics of allele distribution for recessive and dominant traits. A mutation that is beneficial will rapidly spread in its initial phases if it is dominant, but it takes longer to eliminate the less beneficial allele after the dominant allele has spread through much of the population (since selection will have a difficult time acting on the recessive, less beneficial allele, when there is a low probability that it is expressed phenotypically). On the other hand, a mutation that is beneficial but recessive, will take a long time to spread during the initial phases, but will proceed much more rapidly once it has spread to a substantial section of the population. This is because during the initial phases, it will only very rarely be expressed in the phenotype of an organism. However, even a trait that has only a small benefit will tend to spread throughout the entire population after sufficient generations. New mutations will not be lost after one generation.

Another conclusion: “transformation change of a freely crossing population—species, the replacement of the less adapted form by the more adapted one, in a word, the process of adaptive evolution of the species, always proceeds to the end… This conclusion is very important for an accurate understanding of the role of various features in the evolutionary process. Under conditions of free crossing, that is, until there is isolation…the struggle for existence and natural selection can continuously alter the physiognomy of the species, can disseminate more and more new adaptive characters through the whole mass of individuals of the species, can perfect any features of its organization, but never under these conditions does the species give rise to a new species, never will there be a subdivision of the species into two, never will speciation occur.” If selection should cease to operate on the trait while the process is not complete, this will result in stable polymorphism.

“We noted above that the role of free crossing in the process of evolution is a conservative one, striving to maintain the status quo, whereas natural selection acts as an opposing, dynamic factor. But if we bring into the scope of our analysis the process of continuous origin of new mutations as well, then this concept needs to be both changed and supplemented. While free crossing, storing and preserving within the species all the newly arising mutations, gradually unfixes the characteristics of the species, makes it less stable, and produces intraspecific differentiation, natural selection, on the contrary, preserves the stability of the species, its monomorphism. Removing and gradually eliminating all mutations, which in the last analysis appear to be harmful, natural selection purifies the species of contamination by accumulated variations, and, in the case of favorable changes, spreads them to all the individuals of the species, thereby reimposing on it homogeneity.”

On “adaptationism”. “Systematics knows thousands of examples where the species are distinguished not by adaptive but rather by neutral (in the biological sense) characters, and to try to ascribe adaptive significance to all of them is work which is as little productive as it is unrewarding, and in which one does not know at times whether to be more surprised by the boundless ingenuity of the authors themselves or by their faith in the limitless naïveté of their readers.”

“Not selection, but isolation is the actual source, the real cause of the origin of species.”

“Finally, even in those cases in which we are in a position to establish the presence of truly adaptive differences between species, genera, etc., it is necessary to be very cautious in accepting the idea that these differences are of a primary character, i.e., in recognizing that precisely they have given rise to a splitting up of the initial single form into two, thus leading to the process of speciation. It should not be forgotten that, as we just saw, every species in the course of its existence obligately undergoes as a whole Waagen’s adaptive mutation, should a favorable mutation, in our sense, arise, and thereby acquires an adaptive trait absent in its kin. A new species-distinguishing adaptive trait is established, but it is not the cause of the splitting-up of close forms, but on the contrary, its species-characteristic nature is a result of still earlier inter-specific differentiation.”

4. Genotypic milieu

Unity of the genotype, Genotypic milieu, pleiotropy. “The concept of pleiotropic action of genes consists of the idea that every gene may influence not only the specific character corresponding to it, but a whole series of others; generally speaking, the entire soma. In so far as we now accept the proven localization of genes in the chromosomes, and in so far as all cells of the body receive the full set of chromosomes, so in the ultimate differentiation of the cells determining some specific trait all genes can be influential, affecting by their action one or another form of manifestation of genes specifically corresponding to a trait.

“In this way, the former notion of the mosaic structure of the organism consisting of various, independent characters, conditioned by various, independent genes, is discarded. The genes remain pure and qualitatively independent of each other, but their manifestation, that is, the traits they condition, are now a complex result of the manifold interaction of all the genes comprising the genotype of the organism. And each individual is in the literal sense an “in-dividuum”—not divisible. It is not divisible not only in its soma, not only in the physiological functioning of its various parts, but indivisible in the manifestation of its genotype, its hereditary structure. Each inherited trait, the hereditary structure of each cell of its body, is determined by not just some one gene, but by their aggregate, their complex. True, every gene has a specific manifestation, its “trait.” But in its expression this trait depends on the action of the whole genotype.

“Each gene does not act isolatedly from the whole genotype, is not independent of it, but acts, manifests itself, within it, in relation to it. The very same gene will manifest itself differently, depending on the complex of other genes in which it finds itself. For it, this complex, this genotype, will be the genotypic milieu, within the surroundings of which it will be externally manifested.”