New Lucy

Richard.Zander at MOBOT.ORG Richard.Zander at MOBOT.ORG
Mon Mar 29 11:46:29 CST 2004


Charles Fort (Book of the Damned) pointed out that scientists often protect
their more cherished theories by building a wall that keeps out contrary
data. Both morphologists and DNAers at least sometimes build such walls
(those who don't please raise your hands), and the current thread reflects
this.

A way to bust through the wall between morphologically and molecularly based
theorizing is with the Bayes equation, using the probability of results of
one or the other method as a prior. If the arrangements agree, then the
posterior probability should go up; if they disagree, then it should go
down.

For instance, suppose two arrangments (one morphological and one molecular)
disagree. To calculate the probablity that the morphologically based branch
arrangement is correct, use 1/2 times (1 minus the probabilty of the
molecularly determined arrangment) as a prior, which should be approximately
the probability of the morphological arrangment as determined molecularly.
Use the method in http://www.phyloinformatics.org/pdf/2.pdf for calculating
or estimating both molecularly and morphpogically based branch arrangement
probabilities.

Naturally all traits (morphological and molecular) must be independent, as
Tindall's recent post emphasizes. Differential lineage sorting and other
problems must be addressed, and if significant must reduce the posterior
probability.

I expect that branch arrangements that are already "accepted" will be
amenable to this technique, and most problematic arrangments will remain
problematic. "Rigor" in phylogenetics does not necessarily provide a
reliable solution. A branch arrangment that is intuitively ((AB)C,D) may be
rigorously analyzed and AB may be supported by 5 advanced states, AC by 3
and BC by 2. The arrangement ((AB)C,D) remains equivocal, rigorously
equivocal.



______________________
Richard H. Zander
Bryology Group
Missouri Botanical Garden
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