[Fwd: Re: Probabilities on Phylogenetic Trees]

Byron Adams bjadams at CRCVMS.UNL.EDU
Mon Sep 15 16:31:28 CDT 1997


>At 02:15 PM 9/15/97 -0700, Richard Zander wrote:
>> A series of cladograms,
>>each depending on the one previous to establish a correct outgroup, is a
>>house of cards.

Curtis Clark wrote:

>An outgroup is not necessary to form a cladogram, only to root it.

        I think what you meant to say was that an outgroup is not necessary
to form, but only to root a _phenogram_ or _dendrogram_.  Assuming one uses
cladistic treebuilding methodology, by definition an outgroup is required
(in order to establish character polarity) and the resulting tree is a
cladogram.  I don't think many would argue with me that these (unrooted
phenograms/dendrograms) are not the same thing as cladograms.  Nor would
they argue that somebody who can arbitrarily polarize characters _sans_
outgroup comparison is actually doing cladistics. In this case, I think
RZ's criticism of outgroup choice as commonly practiced (one taxon somehow
= a "group") and its affect on character polarization and tree topology is
valid.  But I would restict this "house of cards" criticism to these cases
only.

byron

______________________________________________________________________
Byron J. Adams
Department of Plant Pathology
406 Plant Sciences Hall
P.O. Box 830722
Lincoln, NE 68583-0722
lab (402) 472 5598
fax (402) 472-2853
http://ianrwww.unl.edu/ianr/plntpath/nematode/badams.htm
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