On the Unity of the Genotype

In his book, Toward a New Philosophy of Biology, Ernst Mayr has an essay entitled “The Unity of the Genotype” that is well worth examining. He summarizes the views of a theoretical lineage (beginning with the Soviet philosopher Chetverikov in 1926) through Mayr himself as follows: “Free variability is found only in a limited portion of the genotype. Most genes are tied together into balanced complexes that resist change. The fitness of genes tied up in these complexes is determined far more by the fitness of the complex as a whole than by any functional qualities of the individual genes.” This view must be counterpoised with the Mendelian conception, which it seems to me finds its most modern expression in the writings of Richard Dawkins, of the independence, the segregation, of genes. Here the gene (however this may be defined, a tricky matter) is treated as an isolated unit, the ultimate unit of selection. Evolution is understood as the changing of gene frequencies in a population.

In contrast, Mayr argues that it is necessary to treat the entire genotype as a unified complex of interacting parts. He traces this back to Darwin’s concept of correlation of growth, formulated in the Origin of Species, as the observation “that the whole organization is so tied together during its growth and development, that when slight variations in any part occur, and are accumulated through natural selection, other parts become modified.” Mayr brings in several different manifestations of the cohesion of the genotype, including Lerner’s concept of genetic homeostasis, the tendency for a population to loose some or most of an artificially selected trait when the selection pressure is removed; and the often-observed narrowness of hybrid zones, in which gene flow between two species that have come into contact with each other does not extend beyond a narrow band surrounding the area of cross-mating.

He draws the following conclusions from these observations: “(1) Since the fitness of a gene depends in part on the success of its interaction with its genetic background, it is no longer possible to assign an absolute selective value to a gene. A gene has potentially as many selective values as it has possible genetic backgrounds; (2) The target of selection does not consist of single genes but rather of such components of the phenotype as the eye, the legs, the flower, the thermo-regulatory or photo-synthetic apparatus, etc. …”

The concept of the cohesion of the genotype is useful, because it helps us comprehend certain features of the biological world that would otherwise be difficult to explain. There are, for example, only a certain limited number of animal “types,” the Bauplane: invertebrates, vertebrates, insects, arachnids, etc., and species within each type share a remarkable degree of similarity (the finger bones in a bat wing and in the hand of a human, e.g.). Why is evolution so conservative? Mayr suggests that one possible explanation is that a major change in the underlying structure of an organism (for example, the addition of a new set of extremities) is usually so disruptive to the expression of the genotype as a whole that it is strongly selected against. “The same phenomenon is illustrated by the gill arches that still dominate the ontogeny of land-living vertebrates,” he notes. “It is obvious in all these cases that development is controlled by such a large number of interacting genes that the selection pressure to eliminate vestigial structures is less effective than the selection to maintain the efficiency of well established development pathways.”

The concept also helps explain the evidence of highly uneven rates of evolution (periods of relative stasis or gradual change followed by relatively rapid change). The unity of the genotype acts as a stabilizing force, resisting major evolutionary change, however this stability can be disrupted in certain situations such as the breakaway of “founder populations” (small populations that are separated from the population as a whole), which are confronted with new environmental conditions. [Mayr was really the first theorist to develop this concept, which he called “genetic revolutions,” though it has since become eclipsed by Gould and Eldridge’s more dubious theory of punctuated equilibria, which it is sometimes argued is in conflict with Darwin’s theory of natural selection].

For the moment I am simply throwing this concept out there, but hopefully I will develop these ideas in future posts. I think that it is highly significant that the lineage emerged first among Soviet scientists in the 1920s, pre-Lysenko and prior to the major phases of Stalinist repression. A serious examination of the work of these geneticists would be well worth the effort.