No such thing as a zebra? Sure there is, it's an African animal, black with white stripes. Or maybe, a white animal with black stripes? But, who's asking damn fool questions, when there's a group of them right there!
Well, who's asking the question is Stephen jay Gould in his essay What, If Anything, Is a Zebra? I wrote about Gould recently in The Science of Writing. It's a simple question which Gould writes about to illustrate a wider point: that there is no perfect way of classifying organisms, even though there must be a "right answer" if only we knew enough about evolutionary descent. In his words,
"The potential dilemma for zebras is simply stated: they exist as three species, all with black-and-white stripes to be sure, but differing notably in both numbers of stripes and their patterns. ... Do these three species form a single evolutionary unit? Do they share a common ancestor that gave rise to them alone and to no other species of horse? Or are some zebras more closely related by descent to true horses or to asses than they are to other zebras? If this second possibility is an actuality, ... there is, in an important evolutionary sense, no such thing as a zebra."
To give an extreme example from human life, all redheads are not more closely related to each other than to their parents and siblings! In fact, Gould concludes that the issue is not proven, that zebras may be a complete group, that the word "zebra" may really mean something precise and biological, but that it may not.
Just to doubly confound us, the same is apparently true of two other common terms - there is certainly no such thing as a fish! Or a tree!
"..trees are not what biologists call a natural group. Whereas other successful groups of plants, such as grasses, all descend from a single common ancestor, the tree form has evolved many times in the course of history; their ancestors were quite unrelated plants."
In fact, there are trees which are grasses, roses, mosses, and clovers, and many, many more.
And to go back to zebras, in How the Zebra Gets Its Stripes, Gould concludes - on evidence from embroyology and pigmentation - that the zebra is a black animal with white stripes, not the other way round.