weights, random data etc.

Richard Zander bryo at AGIS.AG.NET
Fri Mar 14 01:17:43 CST 1997


P. Hovenkamp wrote:
>
> Most people (perhaps not most, but at least a great many) using cladistic
> techniques rely on concordance of characters to test each individual character
> as a hypothesis of homology, and use parsimony as the best way to measure this
> concordance. That is all very well, and in accordance with cladistic theory.
> However, without some indication of the amount of phylogenetic signal in a
> dataset, analysing it this way may be as useful as going into a dark room with
> a stopwatch, a flashlight and a mirror and establishing the speed of light as
> 10 m/s ("currently best estimate given our data").
> P. Hovenkamp
> Rijksherbarium/Hortus Botanicus
> The Netherlands
> hovenkamp at rulrhb.leidenuniv.nl
>
> This space intentionally left blank

--
Although it has been said that good data provide the necessary
phylogenetic signal, thus carefully selected data eliminate randomness,
note that independently arising states (homoplasy) are indistinguishable
from the same states arising from shared ancestors. Phylogenetic signal
of course includes such independently arisen states, but parsimony and
only parsimony is supposed to distinguish the two. I wonder if not the
shortest tree, but the longest of the most-parsimonious trees after
sequential deletion (as per G. Davis' ideas) of characters might best
approximate the "true tree." Assuming that evolution occurred and hard
polytomies are rare, a much collasped tree is not an acceptable model of
past phylogeny. The ideal model is a fully or nearly fully resolved
tree.

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