What, if Anything, Is a Zebra? 
Are the three species a genealogical unit or a disparate 
group of horses with some confusing similarities? 
by Stephen Jay Gould 
Each year, professional scientists 
scan thousands of titles, read hundreds 
of abstracts, and study a few papers 
in depth. Since titles are the com- 
monest, and usually the only, form 
of contact between writers and po- 
tential readers in the great glut of 
scientific literature, catchy items are 
appreciated and remembered, but un- 
fortunately rare. Every scientist has 
his favorite title. Mine was coined by 
paleontologist Albert E. Wood in 
1957: “What, if Anything, Is a Rab- 
bit?” ( Evolution , vol. 1 1, pp. 417-25). 
Wood’s question may have been 
wry, but his conclusion was ringing: 
rabbits and their relatives form a co- 
herent, well-defined order of mam- 
mals, not particularly close to rodents 
in evolutionary descent. (And thank 
goodness for small favors as I write 
this column on a quiet Easter Sunday 
evening.) I was reminded of Wood’s 
title last month when I read a serious 
challenge to the integrity of a personal 
favorite among mammals: the zebra. 
Now don’t get too agitated. I am not 
trying to turn the world of received 
opinion upside down. Striped horses 
manifestly exist. But do they form a 
true evolutionary unit? With “Stripes 
Do Not a Zebra Make” — a quite re- 
spectable title in its own right — Debra 
K. Bennett has forced us to extend 
Wood’s question to another group of 
mammals ( Systematic Zoology , vol. 
29, pp. 272-87). What, if anything, 
is a zebra? 
Since evolutionary descent is our 
criterion for biological ordering, we 
define groups of animals by their ge- 
nealogy. We do not join together two 
distantly related groups because their 
members have independently evolved 
some similar features. Humans and 
bottle-nosed dolphins, for example, 
share the pinnacle of brain size among 
mammals. But we do not, for this rea- 
son, establish the taxonomic group 
Psychozoa to house both species — for 
dolphins are more closely related by 
descent with whales and humans with 
apes. We follow the same principles 
in our own genealogies. A boy with 
Down’s syndrome is still his parents’ 
son and not, by reason of his affliction, 
more closely related to other Down’s 
children, no matter how long the list 
of similar features. 
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. (A fourth species, the 
quagga, became extinct early in this 
century; it formed stripes only on its 
neck and forequarters.) These three 
species are all members of the genus 
Equus, as are true horses, asses, and 
donkeys. (In this essay, I use “horse” 
in the generic sense to specify all mem- 
bers of Equus, including asses and 
zebras. When I mean Old Dobbin or 
Man o’War, I will write “true 
horses.”) The integrity of zebras then 
hinges on the answer to a single ques- 
tion: 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, as Bennett suggests, 
then horses with black-and-white 
stripes arose more than once within 
the genus Equus, and there is, in an 
important evolutionary sense, no such 
thing as a zebra. 
But how can we tell, since no one 
witnessed the origin of zebra species 
(or at least australopithecines weren’t 
taking notes at the time), and the fossil 
record is, in this case, too inadequate 
to identify events at so fine a scale. 
During the past twenty years, a set 
of procedures has been codified within 
the science of systematics for resolving 
issues of this kind. The method, called 
cladistics, is a formalization of pro- 
cedures that good taxonomists fol- 
lowed intuitively but did not properly 
express in words, leading to endless 
quibbling and fuzziness of concepts. 
A clade is a branch on an evolutionary 
tree, and cladistics attempts to estab- 
lish the temporal order of branching 
for a set of related species. 
Cladistics has generated a fearful 
jargon, and many of its leading ex- 
ponents in America are among the 
most contentious scientists I have ever 
encountered. But behind the names 
and nastiness lies an important set of 
principles. Still the clear formulation 
of principles does not guarantee an 
unambiguous application in each 
case — as we shall see for our zebras. 
I believe that we can get by with 
just two terms from the bounty offered 
by cladists. Two lineages sharing a 
common ancestor from which no other 
lineage has sprung form a sister group. 
My brother and I form a sister group 
(pardon the confusion of gender) be- 
cause he is my only sib and neither 
of my parents had any other children. 
Cladists attempt to construct hier- 
archies of sister groups in order to 
specify temporal order of branching 
in evolutionary history. For example: 
gorillas and chimpanzees form a sister 
group because no other primate spe- 
cies branched from their common an- 
cestor. We may then take the chimp- 
gorilla sister group as a unit and ask 
which primate forms a sister group 
with it. The answer, according to most 
experts, is us. We now have a sister 
group with three species, each more 
closely related to its two partners than 
to any other species. 
We may extend this process indefi- 
nitely to form a chart of branching 
relationships called a cladogram. But 
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