What do terms like monophyletic, paraphyletic and polyphyletic mean?

17th July 2003

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What do terms like monophyletic, paraphyletic and polyphyletic mean?


These terms are used to describe groupings of organisms, and indicate the extent to which they can be considered as ``natural groups''. They are best explained using examples, so consider the following family-tree diagram:
          Crocodilia  /
Mammalia    \    Dinosauria
     \       \      /
      \       \    /
       \       \  /
   Synapsida  Reptilia
         \      /
          \    /
           \  /

Here are examples of all three types of group:

So far, so straightforward. The only wrinkle in this scheme is that some workers use the word ``monophyletic'' in a sense that includes what we have described here as paraphyletic groups. These people then use ``holophyletic'' to describe what are usually called monophyletic groups. It's tempting in the face of this ambiguity just to abandon the word ``monophyletic'' and use a holophyletic/paraphyletic dichotomy, but this terminological abuse is probably not widespread enough to merit such extreme measures. It's just something to be on the watch for.

Because clades are so important, there is common notation for specifying them (taken from the Phylocode: see note 9.4.1 in http://www.ohiou.edu/phylocode/art9.html).

These notations are not standard in formal technical literature, but appear frequently on the Dinosaur Mailing List.

It's unfortunate that this notation is so clumsy. The following, more concise, alternative notation is sometimes used (notably in Mike Keesey's admirable Dinosauricon):

As examples of this last, we might describe the informal grouping ``non-avian dinosaurs'' as {Dinosauria-Aves}, and the ``traditional reptiles'' as {Reptilia-Aves}.

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