[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Convergence and Coding Characters



Message text written by INTERNET:qilongia@yahoo.com
> 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.<

        You're a bit off base here, but only because I was taking a few
things for granted in my explanation (such as the nature of streptostyly in
a bird).  The quadrate can only swing forward freely when the bone
immediately reostral to it (the quadratojugal) is gone, so in my example, I
differentiated between having the quadrate swing forward and having a
hypothetical fused quadrate-quadratojugal complex swing forward because in
the former, the quadratojugal has been lost entirely; it isn't present to
fuse!  For a taxon with a streptostylic quadrate (with a lost
quadratojugal) to have given rise to a taxon with a fused
quadrate-quadratojugal complex would require that the intermediate taxa
(which may or may not be known) would have to reevolve the quadratojugal
and then fuse it; in this case whilst simultaneously moving the hinge joint
from the squamosal to the exoccipital (I think I wrote "paroccipital" in my
original post; I meant to say "paroccipital process," but somehow
forgot...good catch!).  As you surmise, the likelikhood of this is small;
more likely the two taxa evolved steptostyly (each with their own
variation) to solve the same problems that having streptostyly alleviates. 
But a blind analyst (the cladistics program) would not be able to
differentiate the two types if we code _only_ for the presence or absence
of streptostyly; coding for the nuances of streptostyly are necessary for
the program to see that they are quite different and more likely convergent
than homologous.


                _,_
           ____/_\,)                    ..  _   
--____-===(  _\/                         \\/ \-----_---__
           /\  '                        ^__/>/\____\--------
__________/__\_ ____________________________.//__.//_________

                     Jerry D. Harris
                 Fossil Preparation Lab
          New Mexico Museum of Natural History
                   1801 Mountain Rd NW
               Albuquerque  NM  87104-1375
                 Phone:  (505) 899-2809
                  Fax: ; (505) 841-2866
               102354.2222@compuserve.com