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Re: BCF and BADD
In the BADD scenarios, all the many adaptations for
> flight evolved independently in theropods for reasons unrelated to flying
> (e.g., large wings with flight feathers as sunshades!), and then, when the
> one group of theropods emerged in which all those adaptations converged, they
> just...flew! At the bottom line, none of the BADD scenarios works. You can't
> fight city hall, and you can't fight _gravity_.
> An arboreal animal can become airborne simply _by letting go of the tree_ (or
> cliff face, if you just can't live with the idea of an arboreal archosaur).
> It stands to reason that arboreal animals will evolve mechanisms to prevent
> falls, to control falls that cannot be prevented, and to relieve the effects
> of impacts of uncontrolled falls. Such mechanisms have a clear and obvious
> selective advantage within that lifestyle. Gliding and flying emerge
> _naturally_ as evolutionary elaborations of such mechanisms; powered flight
> is the ultimate adaptation within an arboreal lifestyle.
Exaptation ("preadaptation") should not be disqaulified so lightly,
particularly for flight. Going from a non-flying form to a form with
wings and feathers big enough for flying, gliding or parachuting runs
into a serious problem, namely that of transitional forms, that would be
as severe, possibly even more severe for the trees down model as the
ground up model.
In other words: you have an arboreal form that doesn't fly. It
might have feathers, but not feathers developed for flight. Yes,
falling to one's death would be a terrific selective pressure, but how
many generations does it take to produce feathers and wings that are
developed enough to do something about the problem by allowing
parachuting or gliding? Those first intermediate forms with feathers just a
teeny bit bigger than that first ancestor are going to be smacking the ground
just as hard. When organisms evolve, they aren't thinking down the road and
working for the day that this or that feature will be developed enough to
serve a particular function. They are concerned with immediate
results. If these proto-birds that happen to have feathers bigger than most of
thier totally non-flying relatives are just as likely to be killed or
maimed by a fall, why would the selective pressure ball get rolling
towards developing even bigger feathers? One group's genes are no more
likely to be selected than the others.
Preadaptation gives a way out of this problem (for those of you
not familiar with this term, it may sound like it means the
development of a feature with a specific function down the road in
mind when the feature gets developed enough, exactly as I
described above. Evolution doesn't work this way. The term preadaptation
actually means that a feature developed for one purpose is found to work all
right for another function it wasn't originally developed for, and then
can be fine tuned along those lines. S.J. Gould and someone else
whos name I can't recall suggested this misleading term be
replaced with "exaptation"). If those feathers originally got
big for some other reason, like display (or insect cathing or whatever), and
then it just so happened that those big feathers also slowed down
a fall, then that arboreal proto-bird would be on to something.
However, this explanation works just as well for the "ground up"
model as the for the "trees down" model. Developing big feathers
for display or insect catching or whatever would be as good a
jumping off point for a crusorial proto-bird as for an arboreal
one. Besides, Just because whatever selective pressures causing a
crusorial animal to opt for flight aren't as life-threatening
as a fall from the trees, that doesn't mean that they weren't
strong enough to cause evolution along those lines.