Breeding Structure of Populations in Relation to Speciation

THE problem of speciation involves both the processes by which populations split into non-interbreedingl roups and those by which single populations change their characteristics in time, thus leading to divergence of previously isolated groups. The first step in applying genetics to the problem is undoubtedly the discovery of the actual nature of the genetic differences aniong allied subspecies, species and genera in a large number of representative cases.. Differences which tend to prevent cross-breeding are obviously especially likely to throw light on the process of speciation, but all differences are important. Our information here is still very fragmentary. We know enough, however, to be able to say that there is no one rule either with respect to cross-sterility or to other characters. In some cases the most significant differences seem to be in chromosome number and organization. At the other extrem-ie are groups of species among which gross chromosome differences Iand even major Mendelian differences are lacking, both cross-sterility and character differentiation depending on a multiplicity of minor gene effects. IIn general, there are differences at all levels (cf. Dobzhansky, 1937). But even if we had a complete account of the genetic differences within a group of allied species, we would not necessarily have much understanding of the process by which the situation had been arrived at. A single mutation is not a new species, except perhaps in the case of polyploidy. The symmetry of the Mendelian mechanism