The definition of species is (thank you Oxford);
A group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus and denoted by a Latin binomial.
On Earth, every species eventually has a common ancestor, so it's actually not as useful as determining seperate species as gene exchange and breeding.
In fantasy, some races don't have common ancestors because they are theoretically created by the same god; but it shouldn't matter, because half-elves, half-orcs, and even half-dwarves are a thing is most settings.
In the Forgotten Realms for example, humans can mate with orcs, elves, ogres, and dwarves, and dwarves can actually mate additionally with haflings and gnomes. In addition, such unions do not create infertile offspiring like mules or ligers. Such mixes can continue to create children.
A geneticist would conclude that despite these races having different origins, they are considered the same species. In fact, they would likely dispute the idea of them having different origins at all, as having similar enough genetics to mate means that their creation was likely not as different as the gods may lead you to believe...