Genetic coherence and genetic separation are the outcome of evolutionary mechanisms which maintain genetic variation within populations through recombination on the one hand, and which divide this variation via speciation between reproductively (recombinatorically) more or less isolated populations on the other. While mechanisms of speciation have received considerable attention in biology, their counterpart, mechanisms of genetic coherence, are addressed only implicitly, if at all. Usually, genetic coherence is intuitively associated with the forces maintaining genetic polymorphisms and thus potential for flexible adaptational reaction of populations. However, so far no models seem to exist which explain the evolution of genetic coherence as the natural counterpart of genetic separation or speciation. In this paper a single-locus model is analyzed, in which a mutant allele is introduced into a resident stable diallelic polymorphism, and where this allele is equivalent to one of the resident alleles in all respects with the exception of mating relations. The conditions for replacement of the resident allele by its selectively equivalent mutant are obtained with reference to the associated mating relations. It turned out that for heterozygote advantage the mutant replaces the selectively equivalent resident allele if it increases the mating preferences for carriers of other alleles. The evolution of lower such preferences requires heterozygote inferiority, which confirms the Wallace effect of speciation (by reinforcement). It is argued that this observation suggests that non-selective constituents of the mating system form the section of the genetic system that is responsible for moderating the genetic load implied by adapting selection while simultaneously securing the adaptational potential embodied in the resident allelic variation. Mating systems thus serve the preservation of adaptability.