What are classification, taxonomy, phylogeny, systematics and cladistics?

27th August 2002

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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 summary.

Wish me luck!

Here are some definitions:

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 meaning''.

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.

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