hall at deleteme.ns.utk.edu
Tue Sep 15 16:31:17 EDT 1998
John Grehan wrote:
> However, I would question the statement that
> "Predation pressure probably drove these mating swarms out of the water,
> where thoracic gill flapping became weak flight became strong flight."
> To be "driven out" implies that they could already get out and fly in the
> first place. As
> far as I am aware there is no empirical evidence of such predation
> pressure. Is it
> possible that they moved out without force being applied?
Sure, it is possible. Anne's appeal is sexual selection is also a
tempting possibility. Perhaps the protoinsect mating swarms occured in
very shallow water, where the males displayed their strength and stamina
by jumping, flipping, and/or flapping out of the water, and sexual
selection pushed the protowings to lift-generating form faster than
might have occurred under only predation pressure or dispersal benefit.
I don't think that the cause is particularly crucial to the hypothesis
that wings evolved from gills; after all, that is more historical than
scientific. It's nice to have a sensible, complete story to go along
with the hard evidence of homology and fossil evidence, but a notion of
why the change occurred is merely historical and difficult or impossible
to prove. We should simply prefer the story which invokes the fewest
and most commonplace assumptions about the protoinsects' environment.
If you don't like to invoke predation pressure, choose another story. I
agree with you, really. It's risky to appeal to predation pressure as a
driving force, because you know for sure that the ancestor was able to
survive prior to acquiring the evolved character.
Maybe the last instars crawled ashore to molt into adults just because
it felt good to dry off or feel the Sun. Maybe they flapped their wings
initially to make the buzzing noise that is known to induce willingness
to mate in the females of some insects. I don't know what drove them
out of the water or made them flap their wings/gills in the air, I was
just trying to make a sensible guess.
> Aside from its value as a predator-avoidance mechanism,
> >flight may have also allowed these early insects to disperse upstream
> >more easily than they could have in the water.
> But they managed ok before they had wings.
They sure did. That doesn't mean they couldn't improve though.
> Similarly, during the
> >invasion of stagnant waters, the protoinsect would become more dependent
> >on crawling or swimming to get around, whereas its predecessors might
> >have relied on tides and currents to get around.
> Were the insects necessarily "invaders"?
Well, at some point, yes, but again, that is historical speculation. We
do have fossil evidence of swamps without insects followed by swamps
with insects, so that suggests an invasion. Invasion of new niches is
also known to precipitate adaptive radiations, which rapidly produce a
diversity of forms. The origin of dragonflies, stoneflies, caddisflies,
and others looks a lot like such a radiation in the fossil record to the
extent that I know it.
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