Geographic barriers are an adequate method of segregating species, thus preventing them from interbreeding. However, if such a barrier does not separate two similar species, reproductive barriers must be implemented. Reproductive isolation prevents populations belonging from a different species from interbreeding, even if their ranges overlap. The two categories of such segregation are prezygotic and postzygotic. The difference between them depends upon whether or not they function before or after the formation of a zygote.
Prezygotic barriers impede mating between species or hinder the fertilization of ova if members of different species attempt to mate. These barriers include five types of isolation "habitat, behavioral, temporal, mechanical, and gametic. Habitat isolation involves the separation of species that live in the same area, but encounter each other rarely. Behavior isolation includes special signals that species emit to distinguish themselves from other species, thus reducing confusion between similar species. When two species mate at different times of day, different seasons, or different years, the mixing of genetic material is isolated through temporal isolation. Sometimes, different species may try to mate, but cannot conceive due to structural differences that are not compatible due to mechanical isolation. Finally, gametic isolation provides that the gametes of different species will not fuse to form a zygote.
If the sperm of one individual of a species does manage to fertilize the ovum of an individual from another species, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult. Postzygotic barriers come in three forms "reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. Usually, if two incompatible species mate, the zygote will spontaneously abort. However, in the case of reduced hybrid viability, the zygote will generally not complete development, and the ones that do are in a frail, weakened state.
