tree information content & outgroups (was Re: [Fwd: Re: Probabilities on Phylogenetic Trees])

Byron Adams bjadams at CRCVMS.UNL.EDU
Mon Sep 15 18:39:33 CDT 1997


Byron said (concerning cladograms and rooted trees):
>        I think what you meant to say was that an outgroup is not necessary
>to form, but only to root a _phenogram_ or _dendrogram_.

Tom replied:
>No, it has nothing to do with a phenogram or dendrogram, what is
>rooted is a *network*.

Tom infers fundamental differences in information content among unrooted
dendrograms, including phenograms (contra *networks*).  Sorry to have left
that one out.  My point was that even if one were to root a phenogram, it's
still not a cladogram, because the process of construction and information
content is so fundamentally different.  The distinction between a cladogram
and a network is no exception.

Byron said (re: rooted trees, outgroups, and cladistics):
>>{nor would} they argue that somebody who can arbitrarily polarize
>>characters _sans_
>>outgroup comparison is actually doing cladistics.

Tom replied:
>Using the ontogenetic criterion instead of outgroups is certainly
>accepted within cladistics.

I agree.  But I don't find the ontogenetic method arbitrary, and in the
cladistic process of treebuilding its role in character polarization is
indistiguishable from outgroup comparison.  My point was that due to the
process of character polarization (outgroup comparsion, yes, including the
ontogenetic criterion) the information content of cladograms differs from
other "tree representations", and that the choice of a single taxon as an
"outgroup" can have the dangerous consequences which RZ is referring to as
a "house of cards."  I think you would agree with me that the "house of
cards" argument fizzles under rigorous outgroup (and yes, even ontogenetic)
character polarization.

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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