hair as a shared derived character to 
unite the two mammals and to elimi- 
nate the fish. 
For zebras, the question then be- 
comes: Are stripes a shared derived 
character of the three species? If so, 
the species form a sister group and 
zebras are a genealogical unit. If not, 
as Bennett argues, then zebras are a 
disparate group of horses with some 
confusing similarities. 
The method of cladistics is both 
simple and sensible: establish se- 
quences of sister groups by identifying 
shared derived characters. Unfortu- 
nately, conceptual elegance does not 
guarantee ease of application. The 
rub, in this case, lies in determining 
just what is or is not a shared derived 
character. We have some rough guide- 
lines, and some seat-of-the-pants feel- 
ings, but no unerring formulas. If de- 
rived characters are sufficiently “com- 
plex,” for example, we begin to feel 
confident that they could not have 
evolved independently in separate lin- 
eages and that their mutual presence 
therefore indicates common descent. 
Chimps and gorillas share a set of 
complex and apparently independent 
modifications in several of their chro- 
mosomes (mostly “inversions,” liter- 
ally, the turning around of part of 
a chromosome by breaking, flipping, 
and reattaching). Since these chro- 
mosomal modifications are complex 
and do not seem to represent an “easy” 
modification so adaptively necessary 
that separate lineages might evolve 
them independently, we mark them 
as shared derived characters present 
in the common ancestor of chimps 
and gorillas, and in no other primate. 
Hence they identify chimps and go- 
rillas as a sister group. 
Unfortunately, most derived char- 
acters are more ambiguous. They tend 
to be easy to construct or so advan- 
tageous that several lineages might 
evolve them independently by natural 
selection. Many mammals, for exam- 
ple, develop a sagittal crest — a ridge 
of bone running along the top of the 
skull from front to back and serving 
as an attachment site for muscles. 
Most primates do not have a sagittal 
crest, in part because large brains 
make the cranium bulge and leave 
neither room nor material for such 
a structure. But a general rule for 
scaling of the brain in mammals holds 
that large animals have relatively 
smaller brains than relatives of dimin- 
ished body size. Thus the largest pri- 
mates have a sagittal crest because 
Y)u never forget 
your firstGirl. 
* • 
