Human and ape phylogeny

[Will Fischer]

On Friday, April 4, 2003, at 08:55  AM, John Grehan
<jgrehan at SCIENCEBUFF.ORG> wrote:
> Here's a question for all the cladistic experts on this list... My
> question concerns the 'cladistic' quality of an article cited on this
> list
> as ... support for a human-chimp/african ape clade vs human
> orangutan [ Gibbs, Collard, and Wood (2002) Soft-tissue anatomy
> of the extant hominids... Journal of Anatomy 200: 3-49.]
> ...
> The authors ... state that "Hylobates was assumed to be the basal
> hominoid
> genus and the cladograms were rooted accordingly". Since the characters
> were not defined as synapomorphies with respect to an outgroup, making
> one
> of the taxa the basal lineage seems to be an externally imposed
> criterion
> rather than being generated from the characters themselves...

> Am I correct to view this paper as a 'cladistic' analysis of phenetic
> characters with an arbitrary rooting of one of the taxa being
> analyzed. If
> I am then this paper hardly seems to me to stand up as a reliable
> support
> for the human-chimp clade.

The outgroup choice implies how the cladogram is to be mapped onto
evolutionary history (i.e. ordering the divergences with respect to
time), but doesn't really affect the judgement of relationship.

Gibbs et al. do specifically choose an outgroup (Hylobates as the
outgroup to the remaining hominoids); formally, then, characters for
which Hylobates differs from the other hominoids will be read as
synapomorphies for (Pongo,Gorilla,Pan,Homo).  The hypotheses available
under this assumption are restricted to the 15 possible relationships
of the four ingroup taxa _relative_to_Hylobates_:

Hylobates,(Homo,(Pan,(Gorilla,Pongo)))
Hylobates,(Pan,(Homo,(Gorilla,Pongo)))
Hylobates,(Pan,(Gorilla,(Homo,Pongo)))
Hylobates,(Pan,(Pongo,(Gorilla,Homo)))
Hylobates,((Gorilla,Pongo),(Homo,Pan))
Hylobates,(Homo,(Gorilla,(Pan,Pongo)))
Hylobates,(Gorilla,(Homo,(Pan,Pongo)))
Hylobates,(Gorilla,(Pan,(Homo,Pongo)))
Hylobates,(Gorilla,(Pongo,(Homo,Pan)))
Hylobates,((Gorilla,Homo),(Pan,Pongo))
Hylobates,(Homo,(Pongo,(Gorilla,Pan)))
Hylobates,((Gorilla,Pan),(Homo,Pongo))
Hylobates,(Pongo,(Homo,(Gorilla,Pan)))
Hylobates,(Pongo,(Pan,(Gorilla,Homo)))
---- and the Gibbs et al. tree--------
Hylobates,(Pongo,(Gorilla,(Homo,Pan)))

That being said, recall that the trees are analyzed as unrooted trees,
so (for each of the above topologies) all the alternate rootings are
implicitly considered, and the outgroup choice doesn't affect which two
taxa flank another.  For example, the last of the above trees (the
"standard" molecular view, and that found by Gibbs et al.) could
correspond to any of these seven evolutionary scenarios (depending on
the location of the root, i.e., which lineage was REALLY the first to
diverge):

Hylobates,(Pongo,(Gorilla,(Homo,Pan)))
Pongo,(Hylobates,(Gorilla,(Homo,Pan)))
(Hylobates,Pongo),(Gorilla,(Homo,Pan))
(Gorilla,(Hylobates,Pongo)),(Homo,Pan)
Gorilla,((Hylobates,Pongo),(Homo,Pan))
Pan,(Homo,(Gorilla,(Hylobates,Pongo)))
Homo,(Pan,(Gorilla,(Hylobates,Pongo)))

Pan and Gorilla are the two closest relatives to Homo in each of these
scenarios, unless Gorilla alone is the outgroup, in which case
(Homo,Pan) is supported to the exclusion of (Hylobates,Pongo).

Gibbs et al. say, "The cladogram had a length of 323 ... this cladogram
was 13 steps shorter than the next most parsimonious cladogram, which
linked Gorilla and Pan to the exclusion of Homo, and grouped Gorilla,
Pan and Homo to the exclusion of Pongo."  They also report 95% and 96%
bootstrap support for (Homo,Pan) and (Gorilla,(Homo,Pan)), respectively.

This seems to me to be good support (if the characters are
well-defined) for the hypothesis that chimp and humans are the most
closely related of the great apes.

My only caveat: the authors did set out to test "the hypothesis that
soft-tissue characters can be relied upon to reconstruct hominoid
phylogenetic relationships" -- i.e., they were hoping to be consistent
with the molecular data -- so their character matrix deserves extra
scrutiny, especially w.r.t. the characters supporting the
most-parsimonious tree vs. the set of sub-optimal trees.  Any
anatomists out there who care to comment?

_____________________________________________________________
Will Fischer                       wfischer at uts.cc.utexas.edu

University of Texas at Austin      Lab Ph.: 512-232-7114
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