What are classification, taxonomy, phylogeny, systematics and cladistics?
27th August 2002
What are the differences between all those classification-related
terms? There's classification, taxonomy, phylogeny, systematics,
cladistics and no doubt more. What do they all mean?
It turns out that this is a complicated area, that the lines between
these terms and others are not very clearly drawn, and that people
have very strong, conflicting opinions on the best way to do
classification. From this morass of confusion, I will now attempt to
distil a simple, comprehensive, even-handed and non-inflammatory
Wish me luck!
Here are some definitions:
is a very broad term which simply means putting things in
classes. Any kind of organisational scheme is a
classification: for example, sorting smarties by colour, coins
by diameter or cities by population. Humans seem unable to
resist the urge to classify. It's one of the most basic
activities of any science, because it's easier to think about
a few groups of things than about lots of separate things.
means giving names to things. It tends to go hand in hand
with classification, but need not. You can arrange things
without naming them, or name them without arranging them, but
the most helpful schemes name things in a way the reflects
is the ``tree of life'' - the hierarchical structure by which
every life-form is related to every other life-form.
The word phylogeny refers to the reality of that tree - the
one, true, tree - as opposed to the theories that people make
about it. So phylogeny is not an activity (something that we
do), but a fact (something that we try to discover.)
is the process of trying to classify animals (or plants)
according to their phylogeny. We could describe the
systematics of a group at any moment as being the best current
approximation to its phylogeny - because the phylogeny is a
solid, unchanging thing, but our systematics will change as we
discover more information. (For example, the Therizinosauria
were widely considered to be related to the prosauropods until
the discovery of the basal therizinosaur Beipiaosaurus
showed that they were coelurosaurs.)
also known as phylogenetic systematics,
is a relatively new way of doing systematics. It works by
analysing different taxa to find objective similarities and
differences between them, and using those similarities and
differences to derive a hierarchical structure showing which
taxa are most similar to others. The assumption is that
similar taxa are similar because they are related, so that the
trees produced by cladistic analysis are approximations to the
phylogeny of the group being studied. The cladistic method
was first described in 1966 by Hennig, but has really taken
off on the last decade due to the availability of cheap,
powerful computers to run the analyses.
Unfortunately, the meaning of the word ``cladistics'' is somewhat
muddied by the fact that it seems to carry a philosophy with it as
well as methodolody. That philosophy is that the only
groupings to be discussed are clades - that is, groups consisting of
an ancestor together with all its descendents. So, for example,
cladists do not accept the old concept of ``reptiles'', since it omits
dinosaurs and birds which are descendents of the reptiles as commonly
understood. Whether this is a reasonable stance is a separate issue
(see ``What do terms like phylum, order and family mean?''
but quite what it has to do with the cladistic
method is anyone's guess.
From this ``cladistic philosophy'' come phrases like ``the strict
cladistic meaning of Reptilia'' as opposed to ``the traditional
meaning of Reptilia''. What's meant by this?
When the class Reptilia was first postulated, it consisted of several
groups of animals that were obviously related because of
features such as their scaly skin: lizards and snakes, turtles,
crocodiles, etc. When the dinosaurs were discovered, they were
added to the Reptilia, since they have many skeletal features in
common with other reptiles.
Since then, increasingly sophisticated analyses have shown that the
most recent common ancestor of these groups is also the ancestor of
the mammals (which are synapsid reptiles) and birds (which are
dinosaurs.) It can be said, then, that the group consisting of
lizards, snakes, turtles, crocodiles, dinosaurs, etc. but
not mammals and birds is unnatural (specifically, it is
paraphyletic -- see
``What do terms like monophyletic, paraphyletic and polyphyletic mean?''
Some people argue that such unnatural groups should not be used in
scientific writing, and that the meaning of the word Reptilia should
be changed to include the mammals and birds. This new interpretation
of the Reptilia is sometimes referred to as the ``cladistic
Analogously, now that it is more or less established that birds are
dinosaurs (but see
``Is there any remaining doubt that birds are descended from dinosaurs?''
most scientists prefer to use the ``cladistic'' interpretation of the
class Dinosauria, which includes the birds.
Once again, this dispute about ``cladistic'' terminology has little or
nothing to do with the process of cladistic analysis and the
derivation of putative phylogenies.