Modern taxonomists also consider genetic makeup and functional and behavior features when describing species. The biological species concept emphasizes reproductive isolation In 1942, Ernst Mayr proposed the biological species concept. B. Prezygotic and postzygotic barriers isolate the gene pools of biological species Any factor that impedes two species from producing viable, fertile hybrids contributes to reproductive isolation. Most species are genetically sequestered from other species by more than one type of reproductive barrier. Only intrinsic biological barriers to reproduction will be considered here.
Reproductive barriers prevent interbreeding between closely related species.
The various reproductive barriers which isolate the gene pools of species are classified as either prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes. Prezygotic barriers impede mating between species or hinder fertilization of the ova should members of different species attempt to mate. In the event fertilization does occur, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult. Two species living in different habitats within the same area may encounter each other rarely, if at all, even though they are not technically geographically isolated. Since these two species live primarily in separate habitats, they seldom come into contact as they are ecologically isolated.