A priori homology (was positivism)

[MAGarland]

Doug Yanega and Tom DiBenedetto responded to my post of this quote from a
review by Sneath of the development of numerical taxonomy (sensu lato) :

>"The rise of interest in Hennigian cladistics...was to me very surprising.
It
>seemed obvious that if one could be certain of evolutionary homologies and
>ancestral and descendant character states then the reconstruction of
phylogeny
>would be straightforward."

Tom:
> 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? 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?

>"But I found it hard to believe that these
>homologies and states could be determined in the naive fashion that was
>proposed."

Tom:
>I'm not sure what he is referring to here. The traditions and
>methodologies of comparative anatomy have never struck me as
>particularly naive.

Doug:
>Did Sneath propose an alternative "non-naive" way of determining homologies
and >states?

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

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


Mark A. Garland
Office of Environmental Services
Florida Department of Environmental Protection
3900 Commonwealth Boulevard, Mail Station 140
Tallahassee, FL 32399-3000