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Re: flapping from gliding



In a message dated 96-10-06 18:19:54 EDT, achut@troodon.isp.net (Achut Reddy)
writes (quoting Nick Longrich):

> >      The point: The arboreal hypothesis has two main strengths. a)
> >the leap between gliding flight->powered flight seems to me to be,
> >both conceptually and evolutionarily, an easier one, than the one
> >between a running or leaping predator and powered flight.
> 
> I think this alleged "conceptual ease" exists only if you are already
> predisposed to believe this idea.  To me, the evolutionary path from
> running to wing-assisted running to full powered flight seem just
> as conceptually easy, in fact more so.>>

It is "conceptually easy" because the force of gravity pulls downward, not
upward; the energy keeping a gliding creature aloft comes from its fall
through the earth's gravitational field (in still air, that is). Arboreal and
other acronomic animals require mechanisms to keep themselves from injury
from falls, and every arboreal theory of the origin of flight starts out from
this point. Cursorial flight-origin theories, however, invariably work
_against_ gravity; there is no way to keep the animal aloft unless it already
has the very wings you're trying to get it to evolve.

So, I can't see how you could possibly think that evolution of flight from
the ground up is "conceptually easier" than evolution of flight from the
trees down.
 
> ...Consider extant flightless birds. I believe all of them are good
> runners.  If you believe that modern flightless birds are descended
> from previously flighted forms, then you acknowledge that the
> reverse transition has taken place.  Why then is the transition in
> the other direction conceptually hard?

Some transitions are simply irreversible, for all practical purposes. If you
drop an egg from the breakfast table onto the floor and it breaks, this
doesn't mean the reverse process--in which the eggy mess spontaneously
collects itself into a whole egg and then jumps back onto the table into your
plate--is equally likely! The problem has to do with entropy and energy flow:
the incredible unlikelihood of a random sequence of evolutionary events
backwardly retracing the precise path through a nearly infinite-dimensional
morphological space along which a flightless form evolved from a flying form.
Evolution follows the laws of physics, just like all other physical processes
in this universe.
 
> Besides, pterosaurs must have descended from walkers/runners, not
> gliders.  The further back you go in the pterosaur family tree, the
> better adapted they are for running... One of the most primitive
> pterosaurs known, Preodactylus, has long hind limbs and was
> well-adapted for walking if not running.  Scleromochlus, considered
> by many to be the dinosaur/pterosaur ancestor.  It was not capable
> of flight, but seems to have been adapted for jumping and hopping.
> It could not have been a glider since it lived in a non-arboreal
> desert environment.

We do not know how pterosaurs evolved, just like we do not know how birds and
bats evolved. In fact, we do not yet know just how well pterosaurs were
adapted for walking and running; as with birds, it is more likely that
pterosaurs evolved from climbing forms, not running and jumping forms. And
_Scleromochlus_ could well have been a flightless form descended from
climbing and gliding forms ancestral to pterosaurs.

It is entirely incorrect to state that "the further back you go in the
pterosaur family tree, the better adapted they are for running." There is no
evidence for this whatsoever, unless you go way back to the original
quadrupedal forms when they first became arboreal.
 
> >       I think if there's any reason we are so taken with ground-up
> >theories, it's because we're ground living ourselves and that's how
> >we ourselves did it.
> 
> Not at all.

Don't underestimate the power of psychology in the establishment of theories,
and in the abilities of some wrong theories to be very convincing when
correct theories are counterintuitive.

And, incidentally, humans are considered to have descended from arboreal
forms. This ultimately explains why we have grasping forelimbs and have
evolved the ability to walk on two legs, just as BCF explains why theropods
and other dinosaurs had grasping forelimbs and walked on two legs.