The Evolution of Dominance in Certain Polymorphic Species

Publisher Website
Google Scholar
Abstract
Polymorphism in wild populations must usually imply a balance of selective agencies, of which the simplest type is a selective advantage of the heterozygote over both homozygotes. Such a condition should not be confused with the maintenance of a rare mutant type against counter-selection by means of repeated mutations. While such mutations should on the theory of the selective modification of dominance tend to become recessive, heterozygotes in polymorphic species will tend to resemble in external appearance whichever homozygote it is most advantageous to resemble. The selective balance must then be maintained by some constitutional disadvantage of the homozygous dominant. Nabours'' experiments with the grouse locust Apotettix do, in fact, show such a deficiency of homozygous dominants as is required by this theory. The average amount of the deficiency is about 7%. In 6 individual cases the deficiency is statistically significant, and 6 more show a non-significant deficiency, against 2 showing a non-significant excess. The incidence of dominance and linkage in the fish Lebistes reticulatus strongly suggests that the color genes found by Winge are advantageous in the [male] but disadvantageous in the [female] . The association of the 3 peculiarities of polymorphism, close linkage and the universal recessive type of dominance is found in mollusks, arthropods and vertebrates. It is tentatively suggested that, at least in the grouse locusts and the snails, the primary cause of the two other phenomena may be found in the closeness of linkage within or between chromosomes. This condition presents an obstacle to normal evolutionary development by gene substitution, and so makes it possible for abnormalities such as duplications to possess[long dash]1018[long dash]occasional advantages, so setting up the stability of the gene-ratio necessary for polymorphism; if the advantage lies in the external appearance, the polymorphism will be manifest, and the variant form will tend to become dominant.