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Re: Convergence and Character States



Jerry D. Harris wrote:
>Message text written by INTERNET:znc14@TTACS.TTU.EDU
        That's me, for those of you who don't know... :)

>        This may be fine as far as this goes, but I would hope that anyone
>looking more closely at nuances of anatomy ought to see that this
>bipolarity of data classification is unsatisfactory.
        My morphometrics teacher likes to say that "any discrete character
is a continuous character in disguise." Not sure I agree, but there it is

>As Tom and Chris have
>both said at various times on the list, the simple division of character
>states into two types, when with a little more investigation, many are
>possible, makes for overly-simplistic analyses.
        Certainly. There are many, many cases I've found (mostly in my own
piddling around) where multistate characters are not only the right thing to
do, but, as the saying goes, a tasty way to do it, in that they allow you to
better organize discrete character data and reduce the overall number of
characters.
        One of my biggest problems is with the like of the following
characters (based on an actual paper):
        1       "Tail:          0 full length, 1 with pygostyle"
        2       "Pygostyle:     0 with <5 vertebrae, 1 with >= 5 vertebrae"

        So, how do you code _Velociraptor_? Most people seem to want to code
it "00". But, how do you *know* which state the pygostyle exhibits is it
ISN'T THERE? Sure, of course, most people will say, "the animal never had
one, so it should be the primitive state." Well, how do you know it never
had one? You're a priori weighting your coding. It could have had one, had
one with 8 vertebrae, then redeveloped a full-length tail. It could happen.
So you should code it "0?".
        The better solution, in my mind, is this:
        1       "Tail: 0 full length, 1 with pygostyle<5 verts, 2 >=5 verts/"
                This is an ordered character

        Now you shouldn't be able to ralph it up. Some might suggest that
chaging 2 to:
        "Pygostyle:     0 absent, 1 with <5 vertebrae, 2 with >= 5 vertebrae"

        Will work. My problem is that, then, a reversal to lack of pygostyle
requires 2 steps (character 1 1->0, character 2 1or2->0). You are
overwieghting the reversal.


>To take another example:
        Now, lemme get this straight, you're getting on my back because I
posted a gut-simple cladistics exercise on a completely different topic and
you feel you have to nitpick the details?
        Very me-ish. I wholeheartedly approve! :)

>0 = unfused
>1 = fused only proximally
>2 = fusion proceeds from proximal to distal end through ontogeny
>3 = fusion only distally
>4 = fusion proceeds from distal to proximal end through ontogeny
        Assuming, of course, that this is unordered. "Y"-shaped other other
such advanced character transformation trees would be potentially useful
here. On the other hand, that would involve at least some a-priori assumtion
of evolutionary processes (e.g. that you don't get proximo-distal fusion
without first having just proximal fusion). I have buggered with this
particular character a lot. Not sure I ever reached a satisfactory conclusion.

>etc.  We could also add character states combining the number of original
>metatarsals coupled with fusion types;[...]
        Just please be very careful that your character selection and coding
are not accidentally implying, refuting, or overwieghting homologies a
priori of the analysis. Trying to figure out whether they are or not can tie
you in knots, but it is worth it, IMHO.

>        Similarly, one can easily further divide the presence or absence of
>streptostyly by coupling the actual functional movement (that defines
>streptostyly) with other anatomical traits (osteological, myological, etc.)
>that accompany the capacity to attain streptostylic movement.
        As long as you do not legislate the possibility that the movement is
homologous out of the picture. The more detailed you get into this, I am
afraid you run more quickly into the problem that, lacking "transitional"
forms of the character "complex", you will lead to a situation in which it
is impossible to demonstrate the homology of the function within the skull.

>If we
>code only for the presence or absence of streptostyly, we increase the odds
>that the analysis will find that it is present in the common ancestor of
>both squamates and birds
        I don't see this at all. Unless you do not code for the character
state "streptostyly" itself, it pretty much has an equal chance of
transforming at that node whether or not you add more detail through further
potential transformations. It goes without saying that I am very willing to
be proven wrong on this point, but I think I'm right.

>whereas the addition of further detail might reveal
>that streptostyly evolved twice (or more), once in squamates and once in
>avians.
        Again, what concerns me is only that you might actually eliminate
the possibility that it is homologous. Also, I should point out that, in
this case, your adding more detail will not help the cause very much at all,
since all you are doing is adding more autapomorphic characters for
squamates and birds. This will increase the chances that avian and squamate
taxa (if coded seperately) will group with their own kind rather than with
the other. What is important is the level of similarity (expressed in number
of characters) SHARED by both taxa, and how this relates to the characters
they share with other ingroup OTUs.

>        Of course, I'm not suggesting anyone run such single-character
>analyses; of course other character states would round out an analysis and
>no one would seriously ever suggest that artiodactyls and birds are that
>close (!); I'm only advocating that the addition of character states to an
>analysis be much more detailed and robust to ferret out cases of false
>homology (and relationship).
        I must point out that multi-state characters are, in many ways the
equivalent of multiple bi-state characters. Therefore, you are essentially
advocating that no one do single character analyses. I agree! :)

        :)
        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