animal legs

[Ken Kinman]

>>At 02:35 PM 7/28/99 -0700, Ken Kinman wrote:
> >While the origins and evolution of vertebrate legs are relatively well
>understood, the legs of invertebrates are a whole different story.  They
>almost certainly didn't evolve to walk on land.
>>Curtis Clark responded:
>>I would surmise from the limbs of coelacanths that the same might be true
>>of vertebrates.

     Excellent point Curtis.  This another example of how the continuity of
evolution sometimes makes communication difficult if we have different
concepts or definitions of when one structure "becomes" another.  When a fin
becomes a leg is largely arbitrary.  Likewise, when certain mammals returned
to the sea, legs evolved back into fins in a gradual manner, with
intermediate structures that were both legs and fins simultaneously.  Legs
can also evolve into wings.  In this context, I think it becomes clear why
we must be careful in how we communicate hypotheses concerning the evolution
of structures like the ligule.  Of course, part of the problem is that in
attempting to communicate our ideas to non-scientists, we sometimes become a
bit teleological in efforts to simplify our explanations.
     Another good example of arbitrariness causing confusion is the term
vestigial.  There seemed to be a bit of miscommunication in the Selaginella
thread because of differing conceptions of what a vestigial structure is.
Deciding at what point in time a structure becomes vestigial is somewhat
arbitrary since this process happens gradually.  (and lest I be
misunderstood, I not using the term arbitrary in any judgmental way, but in
a neutral nonjudgmental fashion).
           ----------------Ken



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