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Re: Archaeopteryx and Flight



>> Once into a tree the glider does not have to
>> generate a large amount of lift to get off the ground (and thus does not
>> need large muscles to do this), it just has to be able to keep it's wings
>> extended and let gravity do the rest.
>
>But doesn't this assume that it knew how to glide first? If you kill
>yourself leaping out of trees _without_ the ability to glide, there isn't
>much evolutionary advantage in it.

Isn't this the whole point of natural selection?? (At least by my (educated
layman's) understanding) If you are being chased up a tree by a predator
you may have a choice between being caught (and eaten) or making a jump for
it. If you have the gliding characteristics of a brick, you go splat. OTOH,
if your feathers or wing membranes are a bit bigger than average (and/or
your arm muscles are a bit stronger) you may just get a bit of lift, make
it to the next tree and survive to breed.

If your children inherit the same proto-wing form they will also have a
marginally increased survival potential, and some of their descendants will
have slightly better proto-wings (law of averages and normal variations of
form within a population).

And so it goes on. Even a 0.1 percent improvement in survival potential per
generation will become significant over a few thousand generations.

>RE. Archaeopteryx: Hmmm, let me try to rephrase my questions on this. My
>question is simply one of whether the evolutionary pressure from an
>arborial gliding existence would tend toward a bird-like wing development
>or a flying squirrel-like skin-flap development. Extant arboreal gliders
>all use the skin flap method. We have no modern analog to show that a
>bird-like wing can be derived from this (or that a bird-like wing will
>even evolve in an arboreal species). None of the modern gliding
>animals is even in an interim state of development <towards> the wing
>structure that birds have.

The natural selection mechanism would emphasise any useful variation in a
structure. In flying squirrels, there was presumably a bit of loose skin or
webbing in the right place. In proto-birds AIUI feathers could have evolved
for some entirely different reason (e.g. thermal regulation) and the first
improvements in gliding or parachuting ability would have been quite
fortuitous. However, once a survival advantage is gained, natural selection
could convert an exadapted feathered arm into a proto-wing.


As a layman and ex-lurker, I hope everybody will be gentle with me as well.


_____________________________
Chas Bedford

chasbed4d@chas.demon.co.uk
chasbed4d@cix.compulink.co.uk