Amongst sexual organisms, species frequently evolve allopatrically or peripatrically (in geographically separate regions) from a common ancestor, so that their original areas of occurrence are not in contact or are in contact only at a place where, because of the presence of a certain barrier in the environment (mountain range, river), there is only very limited gene flow between the two species. The places, where the areas of occurrence of emerging species are in contact and where a certain degree of gene flow occurs between the species, are called primary hybrid zones. Primary hybrid zones are apparently rather rare (Jiggins & Mallet 2000). If this were not true and if substantial gene flow were to occur between emerging species, in most cases a new species could not even be formed or, after formation, would again merge with the original species. However, the areas of occurrence of the individual species are frequently not static and grow larger or smaller or are shifted in response to changes in the natural conditions. Thus, it can occur that, in time, two species, that were not originally in contact or were in only very limited contact, come into close contact or their areas of occurrence can even merge. Under these circumstances, there are three possible consequences. If internal (e.g. ethological or ecological) barriers have formed in the species to prevent crossing, the two species can live sympatrically, i.e. in the same territory, one next to the other, or the ecologically better adapted species can suppress the less well adapted species over a certain area. If no reproduction barriers have been formed in the species, the two species can fuse to form a single species, even if they were not originally sibling species – i.e. species that were formed by a single speciation from a common ancestor. If the species have already formed certain reproduction barriers, the contact of the areas of occurrence of two related sexual species usually leads to the formation of a secondary hybrid zone along the line of secondary contact of the two areas. Crossing of the members of two different species occurs at sites in the hybrid zone and thus their hybrids are encountered here. This zone is usually very narrow but may extend for hundreds of kilometers across a continent. The alleles of the individual genes characteristic for one or the other species penetrate through the hybrid zone into the area of occurrence of the other species to different depths (Fig. XX.7), so that detailed genetic studies generally demonstrate that the edges of the hybrid zone are fuzzy (the zone has a different width for each gene) and asymmetric from the viewpoint of the two species – the alleles of a certain gene of the first species penetrate deep into the area of the second species, while the relevant alleles of the second species do not penetrate at all or penetrate to only small distances into the area of the first species (Gündüz et al. 2001). The depth to which a particular allele penetrates depends on the degree to which this allele is compatible with the alleles occurring in the neighboring species, i.e. the degree to which it reduces the fitness of its carrier when combined with the alleles of the genes of the foreign species. However, other phenomena also contribute to the formation of asymmetry, for example differences in the migration activities of the two species and asymmetry in the reproduction barriers – the males of the first species might be able to successfully reproduce with the females of the second species, but the males of the second species are not capable of successful reproduction with the females of the first species (Tiffin, Olson, & Moyle 2001). Some zones are static and remain in the same place for a long time, while others are mobile and move at a certain rate in both directions. In this case, the area of occurrence of one species will gradually expand at the expense of the area of occurrence of the other species.
At a genetic level, a hybrid zone is typically characterized by unusually high occurrence of some otherwise very rare alleles (Schilthuizen, Hoekstra, & Gittenberger 1999; Schilthuizen, Hoekstra, & Gittenberger 2001). Their elevated occurrence is apparently a consequence of epistatic interactions between the alleles derived from one or the other species. Such a combination of alleles cannot occur outside of the hybrid zone, so that the alleles that function very well for these combinations, i.e. increase the fitness of the carriers of alleles derived from different species, are rare there.