Three-taxon analysis

[John Grehan]

At 12:53 AM 4/8/2003 +0200, René Zaragüeta Bagils wrote:

>Concepts arise indeed de novo, in the mind of systematicians. Concepts are
>ideas. The concept of paraphyly arose de novo in the mind of Hennig, for
>instance, and has still to arise in lots of minds.

Whether it did or not arise de novo with Hennig may be bit tricky to claim 
with impunity. There remains the vexing question (even if some cladists 
would like to dismiss it out of hand) whether Hennig ever read Rosa, and 
apparently Rosa did raise the concept of paraphyly. According to Craw 
(1992) in The Margins of Cladistics, Rosa (1918) explicitly stated and 
discussed the following:

1. Natural groups/taxa should be strictly monophyletic
2. Paraphyletic groups should not be admitted into a natural classifications
3. The principle of dichotomous branching
4. The extinction of ancestral species after speciation
5. The rule of deviation
6. The principle of apomorphic and plesiomorphic branches.

The above is not imply that all the above principles are considered 
important by current theorists, but they were important parts of Hennig's 
original formulation of Hennig's phylogenetic systematics.

John Grehan


Dr. John Grehan
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