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Convergence [was: Re: "The X Digit"...a Pteroid bone??]



Larry Febo wrote:
>Well, I believe that cranial kinesis involves different skeletal structure
>and joints in avians vs squamates, so if you want to lable that as some kind
>of "convergence" OK.
        In phylogenetic systematics, homoplaisy (the occurrance of a common
character state other than through inheritance from a common ancestor,
including covergence, parallelism, and reversal) is not judged a priori, but
determined from the (cladistic) analysis. As such, one codes characters
states as potential synapomorphies, and by mapping characters onto the most
parsimonious tree resulting from the data set, we can determine whether the
character state is homologous or homoplastic.
        The reason we do not a priori code character states such as
streptostyly as non-homologous is, as Dr. Holtz so often tells us, evolution
happens. Just because a particular feature is formed in a different way does
not mean that it is non-homologous. The corny old example is tetrapod limbs.
Lobefins and batwings don't look all that much alike, but they are most
probably homologous. However, evolution has altered the limb of the bat to
the point where it takes some work to establish the homology from an
anatomical standpoint (how much work, I'm not sure... never look at the
things myself).
        Now this does bring up the question fo "what do you call a
character?" Well, the simple answer is, you have to use your own judgement.
remember that characters are artificial constructs which we segregate from
the anatomy of animals (at least, last time I checked they were). To my
mind, it seems best to code *anything* that is a potential synapomorphy. If
it looks the same, seems to function the same, whatever, code it. But I'm
not an experienced cladist, and maybe everybody else knows something I don't.
        So, evolution may change the nature of a character state quite a
bit, and it does little good for us to sit around forever arguing whether
the cranial kinesis of birds and lizards are exactly the same, close enough
to be reasonably considered to be homologous, or whether homology is
impossible. We need to test it. So let's do so.
        The character is "skull", the character state is "does not exhibit
streptostylic kinesis = 0," and "exhibits streptostylic kinesis = 1." The
tree we are using is (turtles, (squamates, (pseudosuchians, (pterosaurs,
(lagosuchids, (ornithischians, (sauropodomorphs, (ceratosaurs, (carnosaurs,
(deinonychosaurs, birds))))))))))

Data Matix :    tu sq ps pt la or sa ce ca de bi
                 0  1  0  ?  ?  0  0  0  0  0  1

        (don't know about pterosaurs and lagosuchids offhand)

        If you assume streptostyly evolved in the common ancestor of
squamates and birds, the character requires 7 steps (one 0-1, and 6 1-0). If
you assume it was derived independantly ("convergent"), you only need two
steps (both 0-1). especially if you don't like reversals, this is clearly
the more parsimonious hypothesis.

        Well, anyway, that's what I think.
        :)
        Wagner
--------------------------------------------------------------------------------
     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
 "Only those whose life is short can truly believe that love is forever"-Lorien