speciation only when "the probability of mating between two individuals depend[s] [solely] on their genotypes, [and the genes are] dispersed throughout the range of the population during the period of reproduction". In essence, sympatric speciation does require very strong forces of natural selection to be acting on heritable traits, as there is no geographic isolation to aid in the splitting process. Yet, recent research has begun to indicate that sympatric speciation is not as uncommon as was once assumed. Syntopy Syntopy is a special case of sympatry. It means the joint occurrence of two species in the same habitat at the same time. Just as the broader term sympatry, "syntopy" is used especially for close species that might hybridise or even be sister species. Sympatric species occur together in the same region, but do not necessarily share the same localities as syntopic species do. Areas of syntopy are of interest because they allow to study how similar species may coexist without outcompeting each other. As an example, the two bat species Myotis auriculus and M. evotis were found to be syntopic in North America. In contrast, the marbled newt and the northern crested newt have a large sympatric range in western France, but differ in their habitat preferences and only rarely occur syntopically in the same breeding ponds. Sympatric speciation The lack of geographic constraint in isolating sympatric populations implies that the emerging species avoid interbreeding via other mechanisms. Before speciation is complete, two diverging populations may still produce viable offspring. As speciation progresses, isolating mechanisms – such as gametic incompatibility that renders fertilization of the egg impossible – are selected for in order to increase the reproductive divide between the two populations. Species discrimination Sympatric groups frequently show a greater ability to discriminate between their own species and other closely related species than do allopatric groups. This is shown in the study of hybrid zones. It is also apparent in the differences in levels of prezygotic isolation (by factors that prevent formation of a viable zygote) in both sympatric and allopatric populations. There are two main theories regarding this process: 1) differential fusion, which suggests that only populations with a keen ability to discriminate between species will persist in sympatry; and 2) character displacement, which implies that distinguishing characteristics will be heightened in areas where the species co-occur in order to facilitate discrimination. Reinforcement Reinforcement is the process by