A priori homology (was positivism)

Tom DiBenedetto tdib at UMICH.EDU
Sun Dec 7 13:45:22 CST 1997

MAGarland wrote:

>> All I said was that we arrive, at the end of our
>>biological studies, with a set of homology hypotheses that we
>>(conditionally) accept. Have you never arrived at the point where you
>>are willing to accept a conclusion (while forever remaining open, of
>>course, to further evidence)?

>Of course--but you first have to guard against accepting your conclusion
>before you run your tests. Otherwise, why test?

I'm not sure I understand. The fact that one runs a further test
seems to me to make it rather obvious that the conclusion is not
accepted in any absolute sense.

>And second, you should actually run tests.
>  I'm not sure that straight Hennigian analyses of
>synapomorphies are tests (I know, I know, you can inundate me with info--I've
>read most of the previous posts, but remain unconvinced).  They may be tools,
>but tools for what, exactly?

I dont know; you are the one calling them tools. I think it is a
test. A test of congruence.

>Sneath was referring to the general problem of determining "what should be
>compared with what."  He admitted this was a problem for phenetics as well as
>for any scientific comparisons.  His view appears to be that Hennigian
>cladistics naively skips over this problem.

And I think that is absurd. Hennigian cladistics was/is traditionally
carried out by people who spend their lives studying the details of
organismic structure.

> Two more quotes:  "to assume that
>one could solve evolutionary homology without even considering general
>homology and, furthermore, to determine ancestral and descendant characters
>states a priori appeared to me impossible, indeed quixotic."

What utter incomprehension of systematics is this? To repeat; most of
the cladists I know (the ones whose work I respect) spend their lives
studying "general homology". That may not be the case for Sneath, but
perhaps that is why he approaches systematics the way he does.

> "One cannot
>reconstruct phylogeny from synapomorphies if one must first know the phylogeny
>to recognize correctly the synapomorphies."

One doesnt reconstruct phylogeny from synapomorphies. One
reconstructs phylogeny from homologies. Homologies that are
corroborated by all of the tests, including congruence are then
referred to as synapomorphies. Synapomorphy, along with monophyly,
are conclusions emerging from the test.

>Unfortunately, Sneath didn't propose a "non-naive" way to determine homology
>in this paper.

gee I wonder why!

> And I think you can guess that he'd favor a straight phenetic
>approach.  For phylogenetic reconstruction, he seemed to favor the dreaded
>statistical phylogeneticists like Felsenstein.

As one who is not really interested in phylogenetic classification, I
dont really think his perspectives are of much value.

Tom DiBenedetto                 http://www-personal.umich.edu/~tdib/
Fish Division                                   tdib at umich.edu
University of Michigan Museum of Zoology

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