Oragnutan relationships

John Grehan jrg13 at PSU.EDU
Tue Apr 9 08:29:19 CDT 2002


>Aren't all synapomorphies "identified a posteriori?"  I don't see how you
>can id
>a character as a synapomorphy unless you already have the phylogeny worked
>out.
>
>Richard J. Jensen

I was using Schwartz's terminology and I appear to have created confusion
by not providing the complete context of his statements which are as follows:

As such, molecular synapomorphy is identified a posteriori, after the
phylogeny is generated, rather than as a result of broad outgroup
comparisons that yield hypotheses of character polarity, some of which may
be in conflict. Molecular synapomorphies,however, can take the form of
either entire sequences of specific sites in sequences that are identified
only as a consequence of the clustering analysis used. Thus, there never is
any character conflict within a particular molecular study, only trees of
differing lengths. Whereas hypotheses of morphological synapomorphy can be
used in attempts to falsify other hypotheses of synapomorphy, no such
methodological procedure can be applied to phylogenies generated from
molecular data because the guiding principle in the latter endeavor is one
of "minimum number of steps" needed to achieve the most parsimonious theory
of relatedness.

In a morphological cladistic analysis, the issues of determining character
polarity are dealt with first and then competing theories of relationship
are generated from the alternative combinations of character states.
Basically, features are investigated prior to the phylogenetic assessment
of taxa. In molecular analysis, polarity is determined by specifying a
particular taxon (and thus its molecular attributes) as primitive relative
to the taxa of interest and then generating a tree based on nested sets of
increasing overall similarity among the non-rooted taxa. Molecular
synapomorphy is subsequently defined in the context of the
already-determined branching sequence.

John Grehan




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