The Evolution of Genetic Systems (1958)
C. D. Darlington

The first factor determining hybridity is obviously variation, for if there is no change in the genes and the chromosomes there can be no hybridity. We find that both intragenic and intergenic mutation are controlled by the genotype. A gene in one species of Gossypium when transferred to another species by crossing has a higher mutation rate.1 The particular unstable genes whose frequent mutation is responsible for white flowered plants becoming flaked with colour are always found to vary in frequency of mutation subject to varying genotypes. And they vary also in the time and place of most frequent mutation. The same is true of structural changes. Any change in the breeding system, towards wider crossing or narrower inbreeding, as we saw, causes an enhanced rate of structural change. We must suppose therefore that particular species or at least breeding groups have genotypic properties in this respect more or less adapted to their needs, although no doubt lagging behind these needs as the genetic system changes.

  1. Harland, S. C. 1936. The genetical conception of the species. Biol. Rev. 11: 409-418.
    Sturtevant, A. H. 1937. Essays on evolution. I. On the effects of selection on mutation rate. Quart. Rev. Biol. 12: 464-467.
    Rhoades, M. M. 1938. Effect of the Dt gene on the mutability of the a allele in maize. Genetics, 23: 377-397.