Logic of cladistics

Mike Zwick mezwick at ucdavis.edu
Sun Jun 12 21:41:16 EST 1994


In article <Cr8GKH.FE7 at zoo.toronto.edu>, mes at zoo.toronto.edu (Mark Siddall)
wrote:

> or does not wish to unecessarily invoke processes... the cladistic approach is
> that the simplest explanation (i.e, parsimony) is the best, when given 
> no reason to choose something more complicated.  Let's say you know a
> bird left Albequerque and is now in Allamogordo.  You didn't see it fly
> so you don't know the path it took.  If you draw a striaght line
> you know that the distance flown at least isn't going to be any shorter.
> But birds don't fly in straight lines... agreed, but of all the infinite
> non-straight approaches, which do you choose?  You can't.  So hypothesize the 
> straight line.  Or if there's a cactus in the beak of the bird and no 
> cacti on the stright route, find the closest cacti to the straight
> line and triagulate.  As Joe Felsenstein pointed out (not in this analogy)
> if the bird is in Heart's Content Newfoundland, the straight line from
> Albuquerque is more likely to be wrong than the shorter line to
> Allomogordo.  Still, we agrue, you're not going to get a more efficient
> explanation.
> So I don't think this is so much an assumption of parsimonious process
> as it is an unwillingness to invoke a priori processes or explanations
> that are untestable or unknowable.

But I think the central question may be:  Is it possible to get other
information about the processes that resulted in the characters you are
observing?  To push the analogy some more, if you have independent bird
watchers tell you that these birds always (or perhaps very often?) fly
through Norfolk, Va, then should you incorporate this information into your
reconstruction of the path?  In this case then, you would know (or be
pretty certain) that the parsimony solution would be wrong.  I think some
cladists (with more extreme views than yours) would suggest that there is
on way to ever know about the processes involved, and thus you should
always rely on parsimony.  

-- 
mike zwick
mezwick at ucdavis.edu
Department of Ecology and Evolution
Center for Population Biology



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