Polymorphism, Linkage and the Blood Groups

In a number of groups of animals extreme polymorphism is found and this is not usually confined to one species but is common to several related ones. Moreover, in a large proportion of these species more of the genes responsible for the polymorphism are closely linked than would be expected by chance. Many of these genes have been shown to be subject to strong selection and its direction and intensity often varies from combination to combination and population to population. Although in insects, snails and fish the polymorphism is characterized chiefly by color-pattern differences, in mammals and birds blood groups are involved. The surprisingly frequent association between polymorphism, linkage and strong selection indicates that there is some causal connection between them and suggests that the linkage has been evolved. It is known that if a species is polymorphic for two pairs of linked genes and if the genes have different selective values in different combinations linkage between them is likely to increase as the result of natural selection. It is suggested that if the genes are on different chromosomes any translocation which moves them on to the same one will increase the linkage between the genes and therefore be at an advantage in this respect at least. Consequently such translocations will, on the average, have a better chance of surviving than any others that may arise. In this way genes which alter each other's selective coefficient will tend to accumulate in one or a few linkage groups and the speed at which this happens will depend in part on the strength of selection. The differences in the rate of the accumulation of linked genes accounts for the variation in the number of linkage groups found between closely related species.