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Re: Parsimony



George Olshevsky wrote:

>This is an interesting principle, of course, and it certainly has its uses.
>But nobody has ever shown that nature obeys the principle of parsimony as
>strictly as the cladists insist it does when it comes to phylogeny. Au
>contraire: Others maintain, with equal justification, that some
>characters--particularly morphological characters--may arise repeatedly and
>independently in multiple lineages (such characters are called homoplasies),
>and that enough such homoplasies can defeat the analysis. So, although one
>can frequently settle on a single cladogram as being "most parsimonious,"
>there is no way to show that this cladogram represents the true phylogeny of
>the group under analysis. Repeating the analysis with more characters has the
>unfortunate consequence that an indeterminable number of new homoplasies may
>be introduced into the study, crippling the refinement that the finer
>analysis is supposed to create.

        This would be true if every phylogeny presented were based on one
single character.  When dealing with just the one, the odds that the trait
is a homoplasy or truly represents a close relationship are probably about
even.  The reason cladistic analysis is conducted with character _suites_,
however, is because it is progressively (as more characters are added to
the suite) statistically unlikely that identical suites of characters could
arise independently in numerous groups.  With a large number of
synapomorphies taken as a whole, it is more likely that they are shared
between two organisms (or groups of organisms) due to true phylogenetic
close-ness, and highly unlikely to be due to convergence.  This isn't to
say that convergence is _impossible_, but that the assumption that is
easier to accept ( = parsimony) is that the suites represent true
relationships.

>We should beware of those who invariably accept cladistic analysis
>ahead of all other kinds of analyses in creating phylogenies.

        Yes, but then again, in science, one should always be on the
lookout for new or previously unseen evidence that would alter the result;
nothing is ever written in stone in science.  8-)  Besides, from what I
understand, a great deal of genetic-based phylogenetic work is beginning to
support some of the grosser (that is, at higher taxonomic levels)
conclusions that have been derived from cladistic analysis, at least for
those organisms that have living relatives from which complete genetic
material can be extracted.  This would imply that the methods applied by
cladistics have a potential for accuracy.  Of course, the argument then
would be whether or not the congruence of cladistics and genetic work on
modern groups also holds true for fossil groups, which I can't argue as I'm
certainly no geneticist!


Jerry D. Harris                       (214) 768-2750
Dept. of Geological Sciences          FAX:  (214) 768-2701
Southern Methodist University         jdharris@post.smu.edu
Box 750395                            (CompuServe:  73132,3372)
Dallas  TX  75275-0395

"Hi there!  I am an _Apteryx_:  a wingless bird with hairy feathers!"

                                                        -- Johnny Hart
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