Final answer:
Reproductive isolation occurs when populations of the same species can no longer successfully mate with each other, often due to physical separation and subsequent genetic divergence. This may lead to speciation, and if these species interact, they might produce hybrids in a hybrid zone.
Step-by-step explanation:
When populations of the same species can no longer mate together successfully, it is known as reproductive isolation. This situation might arise when populations of a species are separated, possibly by physical barriers like rivers or mountains, leading to habitat isolation. As these populations evolve independently due to natural selection, mutation, and genetic drift, they may diverge genetically to such an extent that they are no longer capable of interbreeding. An example of this is a cricket population that becomes divided and subsequently evolves into reproductively independent groups. Reproductive isolation can be a result of various mechanisms, classified as either prezygotic or postzygotic barriers, which prevent successful mating or the production of viable offspring.
In some scenarios, after speciation has occurred, closely related species may continue to produce offspring in what is called a hybrid zone. The long-term outcome of these interactions—reinforcement, fusion, or stability—depends on the reproductive barriers and the relative fitness of the hybrids.