Two populations of the same species over time grow distant from one another. At what point will these two populations be considered different species?

Explanation

Speciation is defined by the biological concept of species, which states that species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Therefore, two populations become distinct species when they can no longer interbreed, not merely because of different diets, physical separations, or long periods of isolation. The correct option (D) highlights the importance of reproductive isolation, which is a key element in the process of speciation. Other options fail to address this crucial factor and are therefore incorrect.