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Re: Polytomy question

An interesting question.

--- Manuel Parrado <meparrado@yahoo.com> wrote:

> On 10/25/05, Manuel Parrado <meparrado@yahoo.com>
> wrote:
> >
> > My question is about polytomy in evolution.
> > Phylogenetic analyses, especially of vertebrates,
> > usually show progressive trees in the sense that
> one
> > lineage splits an changes over time and splits
> again
> > and so forth.  

'especially of vertebrates' because vertebrates are
more routinely subjected to cladistic-systematic
analysis. A useful statistical method with a few
shortcomings. Most notably....

> What I find interesting is that every evolutionary
> change is always modeled as a split.  How about
> species that simply change over time without
> spliting
> into two different lineages?  This is rarely
> captured
> in any phylogenetic tree.  Is it because of lack of
> evidence?  Would we know them if we see them (not me
> of course but you get the point).

... the fact that cladistic analysis cannot explicitly
demonstrate ancestor-descendent relationships. Even if
such a relationship exists (and it is worth noting
that they most certainly must exist) the two animals
would likely appear as sister taxa. 

This is in some respects a 'split lineage', but to
think of a cladistic diagram as a 'family tree' is
misleading. It's a statistical diagram, nothing more:
a split may indicate a shared <unknown> common
ancestor between taxa, or that one of the taxa is
infact, the ancestor of the other. it is not possible
to determine which is which using this method, because
cladistics makes no assumptions of time/age of taxa. 

This is a very real problem, and actually becomes more
of a problem the better the fossil record becomes.
Cladistics is a very useful tool, but it doesn't
produce a 'family tree' in the traditional sense.


> Thanks.
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