Just to level set, what is the definition of "species"?
You'll encounter more than one definition. It's a tad fuzzy because all creatures form a continuum, so where you draw the line between closely-related species can seem arbitrary. Usually, the line is drawn where the two versions don't interbreed, at least not often or successfully, but there are exceptions to that.
As time goes on, and the two groups mutate in isolation, it becomes increasingly obvious where the line should be drawn. It's even more obvious if the intermediate stages no longer exist. But sometimes it's a tough call. This seems to be because the development of new species is an ongoing process.