Positivism vs Realism

Richard Zander bryo at COMMTECH.NET
Fri Dec 12 13:58:51 CST 1997

John Trueman wrote:
> James Francis Lyons-Weiler wrote:
> >>        Parsimony is a criterion, but the belief that
> >>        the criterion will lead to meaningful estimates of the past
> >>        requires a bolus of faith I'd rather not choke on.
> and Thomas Pape wrote
> >I cannot quite understand this scepticism towards parsimony. Why should you
> >EVER choose an estimate of the 'truth' that requires a LESS SIMPLE
> >explanation? How can an estimate which requires MORE AD-HOC explanations
> >ever be superior?
> ... and I agree with Thomas's point, but ...
> What puzzles me most in this and in all such discussions on tree-estimation
> (or reconstruction) versus The Truth is that of all tree estimation
> procedures the only one which regularly causes this level of angst is
> PARSIMONY.  What about likelihood trees or distance trees, doesn't the
> exact same argument apply?  We have a criterion for preferring one tree
> over another (max likelihood) or at least a criterion for deciding which
> tree is the estimated tree (the one produced by the algorithm: NJ or
> whatever).  There still must be what James has called "a bolus of faith" to
> raise that tree to the status of a meaningful phylogenetic estimate.

What's wrong with parsimony? Nothing wrong with parsimony itself, but
that maximizing synapomorphy in a tree is maximized parsimony is
imposition of a terribly simplistic evolutionary theory. One assumes
that there is no convergence other than cladistic convergence. Cladistic
convergene is homoplasy that occurs for enough away that convergence
cannot be interpreted wrongly as due to shared ancestry. Take a true
tree ((a b) b), where character b is an advanced trait, and is evolved
twice patristically near. "Parsimony analysis" necessarily interprets
the true tree as ((b b)a). Patristically nearby homoplasy is verboten in
maximum synapomorphy analysis.

Richard H. Zander, Buffalo Museum of Science
1020 Humboldt Pkwy, Buffalo, NY 14211 USA bryo at commtech.net

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