What is wrong with the “development of isolating mechanisms” (and its special case of “reinforcement”)?

Abstract

The conceptual framework of the “reinforcement” model, which plays an important role in the explanatory schemes of evolutionary theory, were examined. According to the current point of view, the reinforcement (of obstacles to crossing under the action of natural selection when there is repeated contact with hybridization between two substantially divergent forms up to a level that prevents them from crossing, with termination of the initiated hybridization) should be a common or even frequent event. In fact, it turns out to be a rare one; it must be specially sought, as all of the detected events are quite dubious, etc. The empirical data on the pros and contras of reinforcement was analyzed using mostly ornithological (and some other zoological) material. It was shown that it does not, in fact, take place in the vast majority of cases in which the theory predicts reinforcement to occur. On the contrary, something quite the opposite can be observed: not only does selection for the enhancement of incompatibility fail to seal the leaks in the gene pools of forms caused by hybridization in the zone of secondary contact, but a permanent hybrid zone is established, which serves as a channel for gene exchange between the forms. Through hybridization these forms are able to adopt foreign genes without compromising their species isolation. This is achieved through the self-nonself recognition in the population systems of both forms in contact. Within zones of repeated contact, the conditions are usually such that individuals constantly make mistakes, which lead to the formation of mixed pairs and the production of hybrids. Thus, at the individual level, self-nonself recognition often appears to be inefficient. This is especially so at the borders of habitation areals, in the zones of repeated contact, etc. Efficient recognition, which ensures species isolation even under continuing hybridization, occurs when individuals of different origin that are the phenotypically “pure,” hybrids, and backcrosses, are assumed into the spatioethological population structures of both forms, in the case of acquisition of territories by established groups, or during the incorporation of such individuals into flocks or other intrapopulation associations of resident species. The population groupings of either form predominantly incorporate individuals with their own (“pure”) phenotype and reject those with the foreign one, including hybrids with an intermediate phenotype. If the two forms have attained a species level of isolation, the hybrids do not establish stable groups of their own but rather try to insert themselves individually into the parental habitations. This appears to be not as successful as it is in the case of the phenotypically “pure” individuals of these forms. Hybrids are thereby directly excluded from the population systems of both forms, even when hybridization continues and they are produced in large quantities. Those hybrids and backcrosses that became residents, occupied a territory, and successfully reproduced do not differ from individuals with the “pure” phenotype in their survival abilities and reproductive success. It has been shown, that examples of reinforcement are not only rare but can usually be explained in some other way. In particular, the “splitting up” of a population system, which give rise to two forms with different ecologies with the following strengthening/enhancement of divergence through sexual selection, can be better explained by our self-nonself recognition model than by the reinforcement. In light of all of this, the conclusion has been drawn that the model of the reinforcement of incompatibility in zones of repeated contact with hybridization is not supported by the data, as is the character displacement model that follows from it. The reasons for the popularity of this theory apparently consists in its conformity with the philosophy of a more general theory that is agreed upon by most researchers and corresponds, on the whole, to the known facts (the theory of modern evolution synthesis, in this case).