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Re: Syntarsus clarification(?)



At 09:11 AM 9/10/96 -0600, Jeff Martz wrote:

>       The word is "preadaptation", which Gould and Vrba? have renamed 
>"exaptation".  It refers to the development of a feature for one purpose 
>that eventually gets conscripted for soemthing else.  The new term was 
>created because the word "preadaptation" sounds like it is implying that 
>the feature was originally developed so that it could 
>be modified into something else more usefull at some indefinite point in 
>the future.  Evolution does not show this kind of adaptive foresight.  

        In point of fact, the word is "preaption", and it was invented to
replace "preadaptation" for the exact reason which you mentioned.  Actually
the point I was trying to make goes along similar veins, but is not exactly
preaption in the strictest sense ( I am easily confused)..  Someone once
proposed, and I believe it the suggestion was not shouted down (shocking, in
evolutionary theory) that some instances of the inheritance of
characteristics may be actually instances of parallel evolution of the same
character in daughter taxa in parallel, rather than direct inheritance of
the characteristic, due to a genetic or behavioral "predispostion" towards
that character.  I am actually not sure that this really falls under the
sway of preaption, and I am especially not sure if it comes under
exaptation, but it sounds quite possible to me.
 
>      Are you implying that these animals evolutionary heritage gave them
>a genetic predisposition to developing analagous structures?  What sort of
>"genetic predisposition" are you talking about?  Do you mean like a slight
>ridge on the snout of thier common ancestor that would be slightly easier
>to develop into a double crest than a smooth snout?[...]

        More like they got all hot-and-bothered when they saw bigger ridges
of certain colors.  Gregory Paul has noted that all coelophysids seem to
have had low nasal horns.  If this were a sexual display feature, several
coelophysoids may have independantly (and doubtlessly in different fauna)
developed large crests from this blueprint as a way of attacting mates (or
however such things evolve).

>[...]Also, assuming the crests are some sort of display structure,
>I'd think that the likelyhood of such a structure developing
>analagously would be fairly low.  What a species finds attractive in
>way of flashy colors and large display structures is pretty
>subjective. Does anyone have any hard data on how frequently DISPLAY
>analogies appear?
 
       Good question.  Lots of birds have wattles and/or feather crests.
Each one seems to have a different shape and/or color (ornithologists,
forgive me my rash generalizations), and, if I'm not mistaken, similar
species have dissimilar display structures.  I wouldn't be surprised if the
likelihood of a similar structure *increases* with genetic distance (since
Mr. Crow isn't going to be confusing Geraldine Robin for his wife anyway, if
you get my drift).
        I'm not saying it developed analogously from nothing, see above on
coelophysoid nasal ridges.  The point is that no one I've heard of is
questioning the close relationship of coelophysiods, it's just the
family/genus/species phylogeny of the group that is at issue.

>LN Jeff
        Wagner
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