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Re: Convergence and Coding Characters



Jerry D. Harris wrote (in a response to Peter Wagner, which I
gleefully snipped >:) :

<Well, let's take another (!) hypothetical example:  let's pretend
that we have one streptostylic organism, A, in which the quadrate is
mobile and hinged as a joint with the squamosal; organism B is also
streptostylic, but the while the quadrate is mobile, it is fused with
the quadratojugal and the whole assembly forms a joint with the
paroccipital. If we just code for the presence or absence of
streptostyly, the analysis might conclude that A and B were close
sister taxa, indicating that streptostyly arose only once, in the
common ancestor of A and B. However, if we also code for the status of
the quadrate in relation to the quadratojugal, and the nature of the
joint, then the analysis would probably put the relationship of A and
B further apart (theoretically we'd have more intermediate taxa in the
analysis and they'd appear between A and B); we'd have to conclude
that streptostyly arose twice, once in the lineage leading to A and
once to B.>

  My problem with this is that the analysis would actually show that B
had more characters than A. Both have streptostyly, only one has two
additional differences, derived or not (more taxa! more taxa!). This
would suggest, at least to moi, that B was a more derived form of A,
or that A and B share a common ancestor for which streptostyly is a
character and in B, the aquisition of additional distinct morphology
would then be defining characters:

---------
  | 1 2 3
---------
A | 1 0 0
B | 1 1 1

1= quadrate ; 0=not streptostylic - 1=streptostylic 
2= quadrate ; 0=free from quadratojugal - 1=fused to
              quadratojugal
3= squamosal ; 0=fused or locked to exocciptial -
               1=forms joint with exoccipital

  A tree is produced:

       B
      /__3
  A  /__2
   \/__1

  Tell me if I'm offbase here.

==
- Often, it is the man who is brought
  down the path to the end who does
  not see his own steps. -

Jaime A. Headden

Qilong, the website, at:
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