Human and ape phylogeny

René Zaragüeta Bagils rzb at MNHN.FR
Sat Apr 5 11:56:05 CST 2003


Pierre Deleporte wrote:

> set one or several "outgroup" taxa in the series of taxa contained in this
> data matrix (in other words, introduce some putative "outgroup" taxa in
> data matrix)
> this is always possible with only one outgroup taxon, this is not always
> possible if you have set several putative outgroup taxa (if they don't
> appear contiguous on the tree, you will have no unique solution to root
> your tree, and this will indicate that you have made a mistake in your a
> priori ingroup / outgroup delineation).

Sorry, but this is indeed NEVER possible with one outgroup taxon, because
you obtain a basal polytomy (unless you use another patch introduced in
computer programs to resolve the polytomy arbitrarily, not in grounds of
refutable evidence). In your second case, i.e. with no unique solution to
root the tree, arbitrariness simply becomes explicit (the patch does not

> this "arbitrariness" in the choice of outgroup(s) is inherent to all
> cladistic analyses
> so the choice should be argued (explain what external evidence support the
> a priori out/in group decision)

No, the "arbitrariness" in the choice of outgroup(s) is inherent to standard
parsimony, because it doesn't find hierarchies. Three-taxon analysis does
not need outgroups and is a cladistic analysis.


Rene Zaragueta-Bagils
Departement Histoire de la Terre UMR 8569
Equipe SICC (Systematique, Informatique, Cladistique et Chronologie)
Museum national d'Histoire naturelle
8 rue Buffon
75005 Paris - France
rzb at

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