The term "race" in biology is used with caution because it can be ambiguous. Generally, when it is used it is effectively a synonym of "subspecies". (For animals, the only taxonomic unit below the species level is usually the subspecies; there are narrower infraspecific ranks in botany, and "race" does not correspond directly with any of them.) Traditionally, subspecies are seen as geographically isolated and genetically differentiated populations. Studies of human genetic variation show that human populations are not geographically isolated, and their genetic differences are far smaller than those among comparable subspecies.