Evolution of Genetic Systems pp. 127-128 (1946)
C. D. Darlington

Integration of Genetic Systems

It is a third corollary of integration that genetic systems can change while external forms remain the same. This is shown by evolution of sex chromosomes but it can also be shown in other ways. Chromosomes can determine evolutionary discontinuities which are not morphologically visible. Two geographically separated varieties of Hordeum sativum give a vast array of segregation in their progeny which is not seen when parents with similar differences come from the same region.1 The same kinds of gene difference in two species of Gossypium have different properties of dominance.2 Certain cryptic species of Drosophila although scarcely distinguishable in form have chromosomes differently arranged and are inter-sterile.3 All these properties go to show that the genetic basis of form may change although the form itself does not. We need not suppose that the external stability of a Lingula depends on an unchanging complement of genes. The genes must change. Forms and their determinants are not necessarily related in the same way in different species at the same time, or in the same species at different times.

  1. Karpechenko, G D 1935. Theory of Distant Hybridisation. Theoretical Bases of Plant Breeding, vol 1. Leningrad (in Russian)
  2. Darlington, C D 1939 Taxonomic species and genetic systems (In The New Systematics, ed. J S Huxley, Oxford)
  3. Harland, S C 1936 The biochemistry of the individual. Perspectives in Biochemistry, pp. 1-10 Cambridge