[Fwd: Re: Probabilities on Phylogenetic Trees]

Tom DiBenedetto tdib at UMICH.EDU
Mon Sep 15 15:15:39 CDT 1997


Byron Adams wrote:

>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_.

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

 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.

Network building is also part of cladistics, but, yes, a complete
cladogram is achieved once the network is rooted.

>{nor would} they argue that somebody who can arbitrarily polarize characters _sans_
>outgroup comparison is actually doing cladistics.

Using the ontogenetic criterion instead of outgroups is certainly
accepted within cladistics.




Tom DiBenedetto                 http://www-personal.umich.edu/~tdib/
Fish Division                                   tdib at umich.edu
University of Michigan Museum of Zoology




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