THE ESTABLISHMENT OF PREZYGOTIC REPRODUCTIVE ISOLATION IN SOME SIMPLE MODELS.

The probability of fixation of a mutant that causes assortative mating according to the 'mass action model' has been studied. A gene of this kind represents one of the simplest hypothetical mechanisms of prezygotic isolation. Computer analysis by Monte Carlo simulations revealed that the fixation of the mutant is very unlikely unless the population is very small in size and/or the degree of partial assortative mating is very low. The introduction into the biological model of pleiotropic effects of the gene on fitness (viability) can facilitate the process of fixation (even to a very large extent in the case of strong selection in favor of the mutant allele). The most interesting situation, from the point of view of a possible role of the mutant in triggering a process of allopatric speciation, occurs when the effect of assortative mating on gene frequency prevails over the effect of differential viability. In these cases, the probability of fixation of the mutant and the consequent establishment of partial prezygotic isolation shows a pattern similar to that described in literature for the simple case of postzygotic isolating mechanism constituted by a Mendelian factor (gene locus or chromosomal variant) with negative heterosis and selective advantage of the 'new' homozygote over the ancestral homozygote.