Sinh học - Chapter 24: The origin of species

No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers. Typically, these barriers are intrinsic to the organisms, not simple geographic separation. Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges overlap. Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes.