taxacom taxacom at IBSS.DVO.RU
Tue Jan 22 13:19:52 CST 2002

Indeed, operator "non" is not always considered class-defining attribute.
(For instance, the absence of chlorophyll does not define any meaningful
taxon). However, in very many cases it may be such attribute (for instance
seeds may be ribbed or non-ribbed in different species).

As for extensional "definition" of taxa, I have never encountered such thing
in my practice. Any species (genus, etc.) is described on the basis of some
difference(or differences = intension) from another one.


----- Original Message -----
From: "SKALA Zdenek" <skala at>
To: "taxacom" <taxacom at>; <TAXACOM at>
Sent: Monday, January 21, 2002 6:59 PM
Subject: RE: paraphyly

> Larissa Vasilyeva:
> >paraphyly is real only from EXTENSIONAL perspective (contents of taxa),
> while INTENSIONALLY (character
> >aspect) paraphyly is mistake (meaning of terms: Buck, Hull, 1966).
> This distinction fails to be general; in a case of clades with (partly)
> homoplastic synapomorphies and/or reversals are even holophyletic taxa
> defined extensionally. At the same time, the Buck&Hull's distinction
> depends on a very specific concept of intensionality (in short, the
> unary operator "non" is not considered class-defining attribute).
> Zdenek Skala
> skala at

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