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| Chiru |
If there's no clear and universal definition of
what a species is, there's no definition at all of what the larger groupings - families, tribes, genera, and whatnot - are. These days, there is a rule that all such groupings should also be "clades" - that is, all the animals in them are more closely related to one another than to anything outside the group. But that's really all there is; there's no way of saying whether a particular clade should be called a "family", an "order", or not given any named ranking at all.
For instance, this rule tells us that humans must belong to the great ape family, since chimps are closer to us than they are to gorillas, but there's nothing in it to say that gibbons can't also belong. We could, in other words, have an "ape family", and the only reason we don't is that we figure gibbons are sufficiently different from great apes that we ought to give them a family of their own. (In fact, "apes" as a whole are considered a superfamily).
