Evolutionary biology categorizes speciation by two organisms’ ability or inability to reproduce and have viable, fertile offspring. So, donkey and horse make mule but mule infertile, so different species. In biology we align DNA sequences to determine genetic similarity and can make “tree” diagrams demonstrating how closely related two organism might be. We can also look at enzymes two organisms share and the differences in their amino acid sequence but also physical structure but neither of these will tell you about whether two organisms are part of the same species, it tells you a beautiful story about evolution though