distically unrelated horses get black- 
and-white stripes? There are two pos- 
sibilities. Either the common ancestor 
of zebras and true horses had stripes 
and true horses lost them, while the 
three species of “zebras” passively re- 
tained them; or else, striping is an 
inherited developmental capacity of 
all horses and not so complex a char- 
acter as it appears. In this case, several 
separate lineages could acquire stripes 
independently. This second possibility 
not only has some evidence going for 
it, but also raises several interesting 
issues in evolutionary theory. (I shall 
have to defer this subject to next 
month’s column. Unlike other unful- 
filled promises to discuss a tantalizing 
subject “next month,” I will come 
through this time, for the piece is al- 
ready written.) Zebras would then be 
horses that have realized a potential 
pathway of development probably 
common to many or all members of 
the genus Equus. 
This cladistic tale of zebras may 
not hold, but the radical messages of 
cladistic ordering are secure in many 
cases. Some of our most common and 
comforting groups no longer exist if 
classifications must be based on cla- 
dograms. With apologies to Mr. Wal- 
ton and to so many coastal compatriots 
in New England, I regret to report 
that there is surely no such thing as 
a fish. About 20,000 species of ver- 
tebrates have scales and fins and live 
in water, but they do not form a co- 
herent cladistic group. Some — the 
lungfishes and the coelacanth in par- 
ticular — are genealogically close to 
the creatures that crawled out on land 
to become amphibians, reptiles, birds, 
and mammals. In a cladistic ordering 
of a trout, a lungfish, and any bird 
or mammal, the lungfish must form 
a sister group with the sparrow or 
elephant, leaving the trout in its 
stream. The characters that form our 
vernacular concept of “fish” are all 
shared primitive and do not therefore 
specify cladistic groups. 
At this point, many biologists rebel, 
and rightly I think. The cladogram 
of trout, lungfish, and elephant is un- 
doubtedly true as an expression of 
branching order in time. But must 
classifications be based only on cla- 
distic information? A coelacanth looks 
like a fish, tastes like a fish, acts like 
a fish, and therefore — in some legiti- 
mate sense beyond hidebound tradi- 
tion — is a fish. 
No debate in evolutionary biology 
has been more intense during the past 
decade than the challenge raised by 
cladistics against traditional schemes 
of classification. The problem arises 
from the complexity of the world, not 
the fuzziness of human thought (al- 
though woolliness has made its usual 
contribution as well). There are two 
components to our vernacular concep- 
tion of “similarity” between organ- 
isms — and classifications are designed 
to reflect relative degrees of similarity. 
On the one hand, we have genealogy, 
or branching order. Cladistics works 
with branching order alone, rigorously 
excluding any other notion of simi- 
larity. But what about the admittedly 
vague and qualitative, but not there- 
fore unimportant, notion of overall 
similarity in form, function, or bio- 
logical role? The coelacanth, to say 
it again, looks and acts like a fish 
even if its closer cladistic relatives are 
mammals. Indeed, another theory of 
classification, called phenetics — from 
a Greek work for appearance — fo- 
cuses on overall similarity alone and 
tries to escape from the charge of 
subjectivity by insisting that phenetic 
classifications be based upon large 
suites of characters, all expressed nu- 
merically and processed by computer. 
Unfortunately, these two types of 
information — branching order and 
overall similarity — do not always yield 
congruent results. The cladist rejects 
overall similarity as a snare and de- 
lusion and works with branching order 
alone. The pheneticist attempts to 
work with overall similarity alone and 
tries to measure it in the vain pursuit 
of objectivity. The traditional system- 
atist tries to balance both kinds of 
information but often falls into hope- 
less subjectivity because they really 
do conflict. Coelacanths are like 
mammals by branching order and like 
trout by biological role. Thus, the cla- 
dists buy potential objectivity at the 
price of ignoring biologically impor- 
tant information. And the tradition- 
alists curry confusion and subjectivity 
by trying to balance two legitimate, 
but often disparate, sources of infor- 
mation. What is to be done? 
I cannot answer this question, as 
it raises issues of style, mores, and 
methodology more than demonstrable 
substance. But I can at least comment 
on the source of this bitter debate — 
a rather simple point that somehow 
got lost in the heat. In an ideal world, 
there would be no conflict among the 
three schools — cladistics, phenetics, 
and traditional — and all would pro- 
duce the same classification for a 
given set of organisms. In this pipe- 
dream world, we would find a perfect 
correlation between phenetic similar- 
ity and recency of common ancestry 
(branching order); that is, the longer 
ago two groups of organisms separated 
from a common ancestor, the more 
unlike they would have become in ap- 
pearance and biological role. Cladists 
would establish an order of branching 
in time by cataloging shared derived 
characters. Pheneticists would crunch 
their numerous measures of similarity 
in their favorite computers and find 
the same order because their most dis- 
similar creatures would have the most 
ancient common ancestors. Tradition- 
alists, finding complete congruence 
between their two sources of infor- 
mation, would join the chorused har- 
mony of agreement. 
But let the reverie halt. The world 
is much more interesting than ideal. 
Phenetic similarity often correlates 
very poorly with recency of common 
ancestry. Our ideal world requires a 
constancy of evolutionary rate in all 
lineages. But rates are enormously var- 
iable. Some lineages change not at 
all for tens of millions of years; others 
undergo marked alterations in a mere 
thousand. When the forebears of ter- 
restrial vertebrates first split off from 
a common ancestry with coelacanths, 
they were still unambiguously fish in 
appearance. But they have evolved, 
along numerous lines during some 250 
million years, into frogs, dinosaurs, fla- 
mingos, and rhinoceroses. Coela- 
canths, on the other hand, are still 
coelacanths. By branching order, the 
modern coelacanth may be closer to 
a rhino than a tuna. But while rhinos, 
on a rapidly evolving line, are now 
so different from that distant common 
ancestor, coelacanths still look and act 
like fish — and we might as well say 
so. Cladists will put them with rhinos, 
pheneticists with tunas; traditionalists 
will hone their rhetoric to defend a 
necessarily subjective decision. 
Nature has imposed this conflict 
upon science by decreeing, through 
the workings of evolution, such un- 
equal rates of change among lineages 
and such a poor correlation between 
phenetic similarity and recency of 
common ancestry. I do not believe 
that nature frustrates us by design, 
but I rejoice in her intransigence none- 
theless. 
Stephen Jay Gould teaches biology, 
geology, and the history of science 
at Harvard University. 
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