Final answer:
A reproductive barrier preventing closely related species from interbreeding is called a prezygotic barrier, essential to speciation processes, which include allopatric and sympatric speciation.
Step-by-step explanation:
A reproductive barrier that prevents individuals from closely related species from interbreeding is an example of a prezygotic barrier. Reproductive isolation is necessary for speciation to occur. For instance, differences in timing of reproductive periods, known as temporal isolation, can prevent species from interbreeding, as can differences in mating behaviors or physical incompatibilities in reproductive structures.
When species are geographically separated, this is referred to as allopatric speciation, where geographic isolation prevents gene flow and can lead to the divergence of species. In contrast, sympatric speciation occurs without physical separation, such as through polyploidy or behavioral changes within a shared habitat.