Systematics and Evolution 139Thus, by using only the shared derived characters, we can construct a branching
diagram of relationships of any three or more organisms. This kind of diagram is known
as a cladogram (fig. 5.3). Cladograms only make statements about who is related to whom
and show the evidence of that relationship by listing the derived characters at the branch-
ing points, or nodes. The power of cladograms is that they make minimal assumptions
about how, why, and when these changes evolved; they only show the pattern of rela-
tionships and not much more. More importantly, a cladogram is instantly testable. All of
the characters are out on display on the nodes, naked and exposed for scrutiny. Anyone
who wishes to do a better job can immediately look at all the evidence and try to come
up with a better hypothesis by falsifying the existing cladogram. By contrast, the family
trees drawn by the old school of taxonomy prior to cladistics were completely untestable.
Taxonomists would draw up a vague branching diagram of relationships, but there was
no way to see on what data they based their family tree and no way to test it easily with-
out redoing all of their work.
For the uninitiated, this simplistic explanation of cladistic methods seems obvious.
What was all the fuss about? Initially, it was because it was a shocking new method with
unorthodox ideas and assumptions, a lot of new alien terminology (most of which I skipped
for simplicity’s sake), and it was pushed by scientists who were not trying to make friends
but win scientific battles. For a decade, the controversy was loud and bitter, and scientists
often ended up calling each other names and insulting each other, both at meetings and in
print. Most of the early, more controversial ideas either have been accepted by the main-
stream or some of the more outrageous ideas have been quietly forgotten.
But a lot of the controversy involves things that are genuinely novel and hard for many
who were accustomed to the older system to accept. Cladistics is called phylogenetic system-
atics because the cladogram is a kind of phylogeny or family tree of life, and it gives us a
natural branching scheme for classification, the ultimate result of Linnaeus’s work. Cladists
assert that classification should be a strict reflection of this phylogeny and nothing more. Any mix-
ing of other factors (such as ecology) mixes shared primitive and derived characters and
leads to confusion. Thus, many of the groups in the classification schemes we have learned
are natural and phylogenetic, but others are not.
For example, we can draw a cladogram of the tetrapods (fig. 5.4) that shows their rela-
tionships based on hundreds of shared derived characters. This part of the process is not
controversial and is accepted by all scientists. But a traditional scientist might want to cluster
turtles, lizards, snakes, and crocodilians in the Reptilia but not want to place the birds within
the Reptilia. Instead, they usually place birds in their own class Aves, parallel to and equal in
rank to class Reptilia. A cladist, however, would say that defining Reptilia without including
all of their descendants (including birds) mixes ecology with phylogeny and is unaccept-
able. For Reptilia to be a natural group, it must include birds as a subgroup because they are
descended from reptiles. To a traditionalist, it seems natural (and comforting and familiar) to
cluster with “reptiles” all of the four-legged land vertebrates that are sluggish and scaly and
cold-blooded, and to elevate birds to their own class because they have changed so much
from their reptilian ancestors. But this mixes ecology with phylogeny because scales and
cold-bloodedness and the other features that turtles and crocodilians have (but birds don’t)
are primitive for the entire group. To a cladist, those primitive characters are irrelevant to
phylogeny and classification. Birds are a subgroup of reptiles in their classifications, and
crocodiles are closer to birds than they are to snakes, turtles, and lizards. The only natural