genetic vs morphological trace of phylogeny

Wed Apr 14 18:33:44 CDT 2004

John Grehan wrote:

> >>>I thought the goal of a cladistic analysis using parsimony was to
> >>>identify the tree with the minimum amount of homoplasy. If this is not
> >>>the way you reconstruct your phylogenetic trees I guess all the
> previous
> >>>discussions have been a waste of time.
> Wrong! Parsimony finds the cladogram with the least number of steps so it
> can be compared with other 'most parsimonious' cladograms. This is why it
> is referred to as an Occcam's razor. It has nothing to do with the least
> amount of homoplasy. A cladogram with 100 steps can be less
> homoplastic than
> one with 99 steps.
Dead wrong. You may want to check Farris (1983). The logical basis of
phylogenetic analysis. Pp. 7-36 in Platnick and Funk (eds.), Advances in
Cladistics. Proceedings of the second meeting of the Willi Hennig
Society, Columbia University Press. In that paper, Farris extensively
discusses the relationship between homoplasy and steps in parsimony
analysis. Assuming that you use the same data in both cases, a cladogram
with 100 steps cannot have less homoplasy than a cladogram with 99 steps.

Unless you use the term "homoplastic" (and perhaps "parsimony",
"cladogram", and "steps" as well) in a, well, very personal meaning that
eludes me completely and that strongly deviates from current mainstream
practice.  Considering your lack of familiarity with the widespread
system of notation that is used for writing down character state
distributions for morphological characters (as Curtis earlier remarked),
this seems to be a real possibility. Off course you are entitled to such
idiosyncracies, but unless you make your intended meanings clear,
nothing useful is going to come out of this. And communication would be
greatly helped if you would make clear in advance if the meaning of
terms as you use them strongly deviates from current standard use. All
this does NOT mean that current mainstream ideas are correct because
they are mainstream, and that all deviating ideas must be wrong
precisely because they deviate. This is just about communicating clearly
and efficiently.

Jan De Laet
Koninklijk Belgisch Museum voor Natuurwetenschappen

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