Circularity & testing.

A. Contreras-Ramos atilano at IBIOLOGIA.UNAM.MX
Sat Oct 19 13:10:55 CDT 1996


Mark Garland wrote:
>What is it about parsimony that inspires such a religious
>faith in its efficacy?  When people believe in something this strongly,
>you can bet they'll overlook any faults--or explain them away with the
>dreaded ad hoc hypothesis.
It seems to me that parsimony actually makes people leave their
attachments to beliefs behind, sometimes in a painful fashion.  That is,
faith in parsimony may turn out into having to reject what we might
believe is the way things are related (is this a religion of not being
religious?)

> "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.  But I found it hard to believe that
>these homologies and states could be determined in the naive fashion that was
>proposed."
Apparently Dr. Sneath's quotation implies that cladistics had a magic key
to homology and phylogeny.  In my opinion proposing homology is still the
same classical procedure.  What became more straightforward was the
presentation of the data, and so to know what support (based on derived
homologies) such and such grouping had.  Perhaps what has changed is that
characters are (most of the times) subjected to the rigor of parsimony,
and not only to their acquisition of polarity (i.e., becoming primitive
or derived based on outgroup comparison, and then being used to propose
groupings).

 Also, Curtis Clark wrote:
>... it seems to me that implicit
>in a lot of the discussion that I see... is the idea
>that homology is simply a matter of support by the data.  In "old-time
>cladistics", homology was a reality of evolution that we sought to discover.
>And also a hypothesis that we sought to falsify. It's true that congruence
>is consistent with homology, but it can never prove homology. Incongruence
>can falsify homology, however.
Good point.  It appears to me that our notion of homology is changing to
a more dynamic one, that is, once a hypothesis of homology is proposed
(e.g., character state 1 supports the grouping of taxa A, B, and C) it
can be challenged and either supported or rejected (e.g., by adding
characters or taxa and redoing the analysis).  A character may be
homologous within one scenario and a convergence within another one.  But
its status as a reality of evolution does not have to change.  I'll be
quiet now (... promise).
;-)  Atilano.

Atilano Contreras-Ramos
Inst. Biologia, Depto. Zoologia; U.N.A.M.;
Apdo. Postal 70-153; 04510 Mexico, D.F.; MEXICO.
(525) 622-5705,06,12; exts. 286,287 (oficina)
(525) 550-0164 (fax); (525) 568-0213 (casa)
atilano at ibiologia.unam.mx
species at servidor.unam.mx




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