Polytomies

Kip Will kww4 at CORNELL.EDU
Tue Jun 17 08:31:01 CDT 1997


At 11:59 PM 6/16/97 -0400, you wrote:
>Eric Zurcher wrote:

>Well, suppose we are dealing with, say, 19 different characters, of which
>10 support ((AB)C) and 9 support (A(BC)). Or perhaps I should say there are
>branch lengths of 9 in one case and 10 in the other. In any case, ((AB)C)
>appears to be a (slightly) more parsimonious tree. Is this really enough to
>regard the potential polytomy as being "resolved"?  I think that to publish
>only the "shorter" tree in this case is actually misleading. I would also
>think that in a case like this, the "best assessment of the data" would be
>that it was indeed a polytomy, unless and until additional evidence
>suggested otherwise.

I don't see it that way. If your only evidence for showing resolution is
character "number 19" (might be any one of the ten), then that *is* the
evidence. If you suspect that character is misleading (from some outside non
character evidence, say biogeography) then you should discuss that fact. If
you present the resolved,  parsimonious tree ((AB)C), then you have set
forth a hypothesis that can be tested against additional evidence. If you
only present (ABC) what is tested with further study? For that matter if you
think (ABC) is the true relationship why bother looking for more evidence?
Also, (ABC) allows for ((AC)B) as a possibility when the data does not
support such a possibility at all.

I think that even the most stubborn supporter of a polytomy can't ignore
character evidence that groups the taxa. You mentioned earlier that this
begs the question of how many. *One* good (subjectivly considered so by the
expert) synapomorphy unites two terminals versus a third terminal. Both
dichotomy and hierarchy is established. If one is not going to accept the
resolution on the *one* piece of character evidence then you must justify
your choice.  Justification means disregarding one or more pieces of
evidence. We should be careful not to "throw out the baby with the bath water".



Ciao,
kip




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