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

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Here are some definitions:

• Classification 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.

• Taxonomy 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 their classification.

• Phylogeny 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.)

• Systematics 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.)

• Cladistics, 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 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.