Positivism in evolutionary scienc

Doug Yanega dyanega at MONO.ICB.UFMG.BR
Sat Dec 6 18:18:51 CST 1997

Mark A. Garland wrote:

>>  rather homology is assumed because your homology hypotheses have
>>  already been corroborated by all the tests you impose on them in your
>>  work as a biologist (anatomist, behaviorist etc.). IOW, you have
>>  already accepted them as homologies in light of all your biological
>>  knowledge; the test of congruence is merely one last test.
>"Already accepted"?  "Merely one last test"?  Hmmm...reminds me of a passage
>in Sneath (Syst. Biol. 44:286. 1995):
>"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.

If you've read Tom's postings you should know that he understands this,
too, and that the phrase "already accepted" is not intended to imply that
these are no longer hypotheses - just that they have already survived
*some* sort of filtering process before they're even put into the matrix.
Even in a molecular study, one must decide which base pairs are supposedly
homologous (often relying on an alignment program), and work from there, so
there's always some set of homology criteria used before applying whatever
matrix-wide tests one uses, whether or not you choose to call them "tests"
or consider them subjective (e.g. modified distal flagellomere of insect A
is homologous to that of insect B even though the total number of
flagellomeres is different). Like Tom said, the congruence test applied to
the matrix is the last test in a series for each hypothesis of homology.

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

Well, if you honestly feel that there is something wrong with forming
hypotheses of homology and then proceeding to test them, then you'd have to
abandon all techniques of phylogeny reconstruction that assume evolution
occurs, and use pure phenetics. Did Sneath propose an alternative
"non-naive" way of determining homologies and states?


Doug Yanega    Depto. de Biologia Geral, Instituto de Ciencias Biologicas,
Univ. Fed. de Minas Gerais, Cx.P. 486, 30.161-970 Belo Horizonte, MG   BRAZIL
phone: 031-449-2579, fax: 031-441-5481  (from U.S., prefix 011-55)
  "There are some enterprises in which a careful disorderliness
        is the true method" - Herman Melville, Moby Dick, Chap. 82

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