The attitrion of taxonomic expertise

James Lyons-Weiler weiler at ERS.UNR.EDU
Fri Oct 18 12:07:59 CDT 1996


On Fri, 18 Oct 1996, A. Contreras-Ramos wrote:

> >The "best fit" part is circular; the data that are used to choose the tree
> >that provides the best fit are used to test homology (the data).
>
> nature (raw data)   --->   proposed homologies (analysis)   --->
> obtained (not chosen) tree(s) with hypothesized synapomorphies   --->
>

This is a description of the congruence criterion for tests of
homology using new data.  My comments were in response to a more
general statement.  Congruence itself cannot provide a critical test for
homology because conditions exist where 1) congruence through convergence
will be misleading, and 2) incongruence through relatively simple
geneaological processes (i.e., coalescence) results from more than one
true phylogenetic history for different characters.  The accumulation of
more and more data itself is an entirely inductive exercise; how do you
know when you have enough data?  How do know you wouldn't find
incongruence over the next hill?  Such is the natural of pure empiricism,
but tests exist to tackle these problems, and more are under development.

> additional information (e.g., more characters), new assumptions, new

naturally, we are apt to come to disparate conclusions when we change our
assumptions.  Where is the test?

> sources of characters (= testing of proposed phylogenetic relationships
> and so of supporting synapomorphies)

So if I have 80 characters, homology assumed, get a tree (chosen* by the
criterion of maximum parsimony), then I have a phylogeny.  Then I look at
synapomorphies on the tree.  I check out my original assessment of
homology by adding more data; I get a new tree; this is a phylogeny; I
look again at the synapomorphies on the tree, to check my original
hypothesis of homology.  Somewhere in there is a critical test?  BTW,
can't homoplasious character states be homologous?  So how can a
hypothesis of synapomorphy test a hypothesis of homology?  When it comes
down to it, the outcome of the test depends upon which data are collected
first.

James Lyons-Weiler




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