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Re: Parsimony



In a message dated 96-11-06 03:01:52 EST, swf@ElSegundoCA.NCR.COM (Stan
Friesen) writes:

> The parsimony principle states that a phylogenetic reconstruction
> that requires fewer changes (evolutionary steps) is better than
> one which require more steps.
> 
> The way it is used in cladistics is that one constructs a cladogram
> an then counts the number of state changes along the branches. The
> cladogram with the lowest count is the most parsimonious. 

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.

Cladists maintain that the way to discover homoplasies is to perform the
cladistic analysis, then see which characters appear more than once in the
most parsimonious cladogram. Others maintain that certain characters may be
discarded as homoplasies before performing the analysis. Here again, there is
no way to resolve the differences between these viewpoints.

So, while cladistic analysis represents a significant improvement over the
phylogenetic methods that came before, and it even allows us to attach a
numerical figure to the confidence we can have in the truth of a particular
cladogram, it has definite and fundamental limitations. We should beware of
those who invariably accept cladistic analysis ahead of all other kinds of
analyses in creating phylogenies.