holophyly (was: Ashlock was treated badly)

Curtis Clark jcclark at CSUPOMONA.EDU
Tue Jan 8 07:38:20 CST 2002

At 11:48 PM 1/7/02, =?iso-8859-2?Q?SK=C1LA_Zden=ECk?= wrote:
>Not exactly; we are discussing the oriented (rooted) cladogram I
>believe, so each taxon has one origin only (you can speak of the "end"
>of taxon if you want, indeed).

A paraphyletic taxon "ends" when an excluded clade originates. If mammals
an birds had never evolved, reptiles would be monophyletic.

>The paraphyletic taxa are natural/unnatural in the same sense as are the
>holophyletic: human always need to (1) either specify the limit(s) of
>them (well, one for holophyletic, two for paraphyletic ones) (2) or
>accept that each splitting on the cladogram will constitute (real, not
>only possible!) taxa, which, I guess, is hardly used option.

The clade of all flowering plants (if in fact they originated from a single
species) exists without human intervention--in fact, it had existed for
over 100 million years before there were any humans at all.

>There can exist various sets of holophyletic taxa over a cladogram as
>well as various sets of paraphyletic taxa. Why do you think that a
>choice among different holophyletic solutions is less artificial than a
>choice among the paraphyletic solutions? Yes, different para solutions
>can overlap, but the *final*  solutions of course does not include
>overlaps, so where is the problem?

Ignore for a moment the hybrid origin of taxa. Any non-overlapping
hierarchy cleanly contains *every* clade, named and unnamed. It contains
only a limited set of paraphyletic groups.

But I'm not saying that the issue is with overlapping (since after all,
taxa do originate now and again by hybridization). Rather, paraphyletic
groups and clades have a different biological meaning. A classification of
overlapping paraphyletic groups could be as "natural" as one of clades,
perhaps, but mixing them is like selling apples and claiming they are oranges.

Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu

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