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Re: Polytomy question



--- "T. Michael Keesey" <keesey@gmail.com> wrote:

> On 10/26/05, Denver Fowler <df9465@yahoo.co.uk>
> wrote:
> > 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:
> 
> Well put.
> 
> > 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.
> 
> However, you can use a cladogram combined with data
> on the time ranges
> of your OTUs to identify *potential* ancestors. If
> two sister taxa, A
> and B, are scored identically except that A lacks
> certain derived
> traits that B exhibits, and every member of A occurs
> earlier than
> every member of B, then A *might* be ancestral to B
> (although this
> can't be proven).
> --

Yes of course, there are problems though. for example
if you have a single evolving lineage sampled at four
points in time:

Time in capitals, sampled animal in lower case
ABCDEFGHIJK
a c     i k

taxon a is the same population as c, i and k, just
sampled at different points.

if there is a constant rate of evolution (=constant
change in unweighted characters) from A to K then you
might expect a and c to be closer to each other than
either is to i and k, and vice versa:

a c  
| | 
---  i
 |   |
-------k

good huh?

this creates the illusion that we see two separate
lineages whereas all we really see is a measure of
relatedness of a single lineage sampled at uneven
intervals. There are groups of animals that
demonstrate this pattern in the fossil record. Some
people see it is as large-scale diversity patterns.
They should not.

I believe there is alot more ancestry in the
vertebrate record than we might think. Ironically, as
the known resolution of the fossil record improves,
the number of 'intermediates' increases,  and the
boundaries between taxa become less sharply defined,
cladistic analyses become more complicated, not less.

Denver.

> Mike Keesey
> The Dinosauricon: http://dino.lm.com
> Parry & Carney: http://parryandcarney.com
> 



                
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