- As soon as postzygotic reproductive isolation barriers are formed for some reason between various forms in a single population, selection pressure immediately appears for the formation of prezygotic barriers, for example ethological barriers. If a particular mutant gains the ability to recognize whether or not its potential sexual partner is reproductively compatible with it and is capable of preferentially reproducing with compatible individuals, it immediately gains a selection advantage over the other members of the population.
The model of reinforcement of reproduction barriers through such selection (the reinforcement model) is frequently successfully applied to explaining processes occurring during secondary encounter of two populations that developed separately for a longer period of time and between whose members postzygotic reproductive isolation barriers have at least partially formed (Butlin & Tregenza 1997). If genetic variability exists in these populations in preference for sexual partners, the reinforcement mechanism can rapidly complete the formation of interspecific barriers. This mechanism has lower effectiveness for speciation that runs sympatrically from the very beginning. In these cases, the fitness of the less numerous form is fundamentally lower than that of the more numerous form, as its members more frequently encounter genetically incompatible sexual partners. The tendency to reproduce exclusively with the members of one’s own form can, in addition, substantially reduce the choice of potential sexual partners and thus even further reduce the chance of reproduction of individuals with emerging prezygotic isolation mechanisms. The ability to differ between the two forms is understandably also advantageous for the members of the less common form. A weak point of the reinforcement model lies in the risk that, before the reinforcement creates an impermeable reproductive barrier between the two species, the individual genes causing both postzygotic isolation and prezygotic preference for the members of a certain form as a consequence of genetic recombination will end up in the wrong gene pool and will thus reduce the chance of fixation of the genes for preference for one’s own form. However, experiments with artificial selection in favor of drosophila capable of discriminating between their own and foreign forms indicated that the isolation reinforcement mechanism is quite realistic and that sufficiently strong prezygotic isolation barriers can emerge within just a few generations (Rice & Hostert 1993).
The effectiveness of this mechanism has also been confirmed by observations of natural populations. While postzygotic reproduction barriers are equally strong between allopatric and sympatric pairs of drosophila species, prezygotic (ethological) barriers are much stronger for sympatric pairs and are formed in them much faster than for allopatric species (Coyne & Orr 1989). An old species of Galapagan finches, Certhidea fusca, which lives alone on the island, did not form a sufficiently strong ethological prezygotic reproductive isolation barrier even over 1.5 – 2 million years, while much younger species, occurring sympatrically on other islands, formed these barriers (recognition of species-specific song) over a much shorter time (Grant & Grant 2002). Similarly, a meta-study, i.e. a study performed by the methods of statistical meta-analysis on the basis of a great many formerly published works indicated that it holds for the most varied taxons that sympatric species have better developed traits according to which the members of a single species recognize one another and thus better developed prezygotic reproduction barriers than allopatric species (Noor 1999).
It can be objected that the cause of the stronger reproduction barriers in sympatric species does not lie in reinforcement but simply in the fact that species that did not have these barriers fused together and thus disappeared. This explanation is apparently erroneous as, in this case, the differences between sympatric and allopatric species would apply to both prezygotic and postzygotic barriers (Noor 1999).
The mechanism of the reinforcement model is similar to the character displacement model (Schluter 2000). This describes the situation where two species with partly overlapping niches occur together at some places and independently at other places. In these cases, the ecological valence of the two species frequently differs between the two types. At places where the two species occur together and where they thus compete, ecological specialization and thus greater phenotype differentiation occur. In contrast, at places where only one species occurs, they have broader ecological valence and each species utilizes more resources from the environment and their phenotypes are more similar. The character displacement model differs from the reinforcement model primarily in that it can be valid only for reproductively isolated species. Moreover, the reinforcement model concerns the formation of reproduction barriers and not ecological specialization. Last but not least, the two models are based on somewhat different mechanisms (Schilthuizen 2000). While, in the case of reinforcement, individuals with imperfectly developed ability to discriminate between members of their own and a foreign species have lower fertility (because they produce more unviable crosses), in the case of displacement, unspecialized individuals have lower viability, because they attempt to utilize the resources for which the members of another species have specialized and are better adapted.