Answer & Explanation:
The first important part to consider is the actual definition of species. There are many, depending on the field. Essentially two organisms belong to the same species if they can mate, cross and the breeding is viable. This means that, if they do mate and actually give birth, the baby must be able to also mate and breed. Hybrids, for example, cannot breed and that's because the parents did not belong to the same species, causing that the breeding is infertile. Recently molecular studies have been able to recognize organisms from the same species with their genetic similarities or differences.