Computer Simulation Model of a Hybrid Zone: Assortative Mating in the Role of Selection against Hybrids

Abstract

The assumption of low hybrid fitness serves as the cornerstone in the logical justification of the discreteness of species in terms of the biological concept. The majority of natural hybrid zones are narrow and stable in time; thus, this proves, as mathematical models have shown, the existence of selection against hybrids. It is assumed that, since the coadaptive gene complexes of the parent forms are destroyed in hybrids, they must be less viable and/or fertile. However, this statement remains no more than a hypothesis. The low reproductive success of hybrids may have other causes, e.g., they may have difficulties in pair formation because of the rarity and strangeness of their appearance. A computer simulation model has been constructed to verify this assumption. The main purpose of this work is to give answers to the following questions. (1) Is it possible to form a narrow and stable hybrid zone with assortative mating but without lower viability and fertility of the hybrids? Assortative mating in the model is due to sexual imprinting: descendants remember the image of one of their parents and seek an appropriate partner. (2) Is it possible to complete the speciation process under these conditions and what is necessary for this? Here, we interpret speciation as the process of the establishment of sympatry between the initially allopatric forms with the maintenance of their appearance and a low level of hybridization. The model study gave positive answers for both questions. Without biotopic segregation and with assortative mating, the hybrid zone is narrow and stable in time. Hybrids remain unmated more often than parent forms (5–8 and 1–3% respectively), which results in selection pressure in favor of a narrow hybrid zone. In addition to assortative mating, biotopic segregation is necessary to establish sympatry, and hybrids have to be less successful than parent forms in the occupation of an individual territory. In opposite case, when the biotopes of the both parent forms are optimal for hybrids, the contact zone became wider at first and then, after the smoothing of biotopic segregation, they started to slowly shrink to a narrow hybrid zone. As a result, the two forms became indistinguishable by selectively neutral loci (DNA markers analog) over the entire space of the model.