What determines whether two organisms belong to the same species?
If two organisms are capable of producing offspring in nature, the belong to the same species. However, some organisms are capable of crossbreeding but are not the same species.
A population is all the organisms that both belong to the same species and live in the same geographical area. A species is defined as a group of organisms capable of interbreeding and producing fertile offspring. There can be multiple populations of one species but a population only consist of one species.
Donkeys and horses are not from the same species because of this: In sexual organisms, two organisms belong to the same species if they are able to breed and have fertile offspring. Although horses and donkeys can have mules, they are infertile, so they are two separate species and the mule is a hybrid.
Paradoxes: * Two organisms "supposed to be"of the same species, and both masculine, cannot breed together, so they can't be of the same species. * If an organism "A" can breed with two other organisms "B" and "C", it means that "A", "B" and "C" all (appear to) belong to a single species, but not necessarily "B" and "C" must be capable of breed together, meaning that "B" and "C" belong to different species.