The attitrion of taxonomic expertise

Tom DiBenedetto tdib at UMICH.EDU
Fri Oct 18 12:58:40 CDT 1996


On Fri, 18 Oct 1996 12:07:59 -0700, James Lyons-Weiler wrote:
>So if I have 80 characters, homology assumed,

homology assumed,,generality of homology hypothesized

>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;

no you dont "check out your homology assessments by adding data",
your homology assessments are checked out by the test of congruence.
Adding data is done under the assumption that no factor other than
historical descent can pattern characters in all aspects of the
genotype and phenotype; thus by acessing the complete range of
possible characters you are assured of extracting the most general
pattern.

>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?

Not in your "there" perhaps,but you are not describing cladistic
parsimony.

>  BTW, can't homoplasious character states be homologous?

Of course,,they ARE homologous, but at a lower level than orginally
proposed. Matching character-states are generalized to the extent
possible, and the congruence test reveals instances of
over-generlization relative to the remaining data,,,,homoplasy.

>So how can a
>hypothesis of synapomorphy test a hypothesis of homology?

The test is of the generality of the hypothesis; given that
generality of homology is synonomous with the question of identifying
the level at which the novelty arose, the test can be seen as a test
of homology itself. On some level though, all heritable characters
are homologous, even if only with the terminals.

  When it comes down to it, the outcome of the test depends upon
which data are collected
>first.

not at all.

tom
---------------------------------------------
Tom DiBenedetto
Fish Division
Museum of Zoology
University of Michigan




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