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Re: Reversals (was Re: OCCAM'S RAZOR & THERIZINOSAURS)



David Lessin wrote:
As long as we are splitting hairs...
        It seems that everything, especially everything classificatory,
comes down to this eventually. :)

>a true "reversal" sensu-stricto  would be nearly impossible. [...] John
>[sic] does (I assume) agree with this position.
        a) I would argue, from what limited experience I have, that George's
*exact* reversal is NOT the "sensu stricto" of reversal.
        b) You assume correctly. The point of the first half of my posting
on this subject was that I do agree that *exact* reversals of the type
George is discussing do not occur. Nor do I think anyone has ever said they
have occured, but that's another matter.
        c) Jonathan (variant spelling Jonathon, a few abberant spellings) is
not shortened to J-o-H-n, but J-o-n. Except for a few people, but they are
freaks anyway. :)

>We are having a semantics debate. If a character cannot be a point in
>morphospace- if it is to be inclusive of a taxa/clade - it must at least
>be within a discrete range. This is how morphological analysis works. 
        Well, the range is not always discrete, but you can give your
definition of character and states, and show your data. If someone
disagrees, they can change your analysis. My point was simply that since
character states are not points, even using George's "random walk" model
(ill-founded as I think it is), we can return to something within the
morphological bounds of the ancestral character.
        These morphological bounds are set by the observer (in a process of
classification), and they should be set as wide as necessary to account for
the possibility that what you are observing is not a return to the ancestral
state, but something that stayed there the whole time, and just wandered
around a little.

>One question this debate does raise... is there such a thing as genetic
>reversal?
        Personally, I do not know. I believe the topic was discussed in a
class I had a while back, but I've been too busy with geology to spend much
time with biology in the recent past.

>But the question still remains- was (if BCF is a valid theory) this new
>flightlessness based on convergence or reversal?
        The point of my entire last two posts on this subject is that this
is a semantic nightmare. Please please PLEASE save everyone on this list
considerable time and effort and use the "cladistic" definition of reversal
I gave before. Using an operational definition of reversal ensures that the
most people will understand your point. For the past several years, George
and others have fought to a standstill on the "reversal" issue, and it now
seems that most of the meat of those arguments was lost because we were
using different terminology. Please do not perpetuate this madness. 
        In this case, flightlessness in birds constitutes a reversal
regardless of the process by which it developed. As I have already pointed
out, the "genetic" versus "convergent" mechanism for reversal is probably
*very* hard to test. It seems unlikely that we will have means of telling,
especially from the fossil record. Frankly, it seems possible that the two
maechanisms may be inexorably intertwined.
        If you wish to examine this question further, by all means do so.
I'm sorry to ask you to burden your writing with apparently excessive and
proliferate jargon, but please adopt some system such as the
"genetic/convergent reversal" system I have tried to use.

>Rather, selectional stresses may temporarily favor  the release of
>"cryptic" genetic variability.
        Cool!

>What this implies is that neither of the terms , "reversal" or
>"convergence",  fully describe a return to an ancestral condition- since
>it would appear that via meiosis and recombination, new genotypes are
>created from old.
        Well, using the operational definition, "reversal" is the correct
term regardless. Did you just say exactly the same thing I said, only with,
like, science and stuff to back it up? I love it when that happens!
        :)
        Wagner
----------------------------------------------------------------------------
    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                    "...To fight legends." - Kosh Naranek