Distribution of mating activity in five sympatric frog species. Differences in mating seasen prevents gene flow among them.
Differences in courtship behaviour in the sympatric and morphologically similar Hawaiian fruitflies. Females respond only to behaviors characteristic of her own species. Drosophila heteroneura left, Drosophila simulans middle, and Drosophila ?right.
Reproductive structures or genitalia of species are morphologically very different so that fertilization or efficient pollen transfer between the two is not possible.
Individuals of the two species live in the same general area (sympatric), but inhabit different habitats, such as open grassland versus woodland, swampy places versus dry rocks, different soil types, etc. Habitat separation can effectively prevent gene flow.
Gametes can not meet or fuse together due to the lack of appropriate chemical cues.
Hybrid sterility can be due to incompatibility between the genes of two species or to chromosomal differences that prevent normal meiosis. In some cases, species' chromosomes do not pair because of lack of homology. In other cases, pairing occurs but inviable gametes are formed when species differ by inversions and translocations. Inversions involve single pairs of homologous chromosomes, whereas translocations involve two pairs of homologous chromosomes.
Hybrids, like these zebroids (crosses between horses and zebras, shown
among horses) are often viable, but sterile.
Hybrids sometimes can be partially fertile, but their offspring is inviable, or sterile. This prevents the perpetuation of the hybrid line.