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Re: BCF and BADD



In a message dated 95-11-28 22:30:03 EST, martz@holly.ColoState.EDU (Jeffrey
Martz) writes:

>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.

I don't have any problems with exaptation; I have problems with _fifty_ (or
some other large number greater than three) exaptations all appearing
together in one cursorial form.

>     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?

GUARANTEED many fewer generations than in the "ground up" scenario!

>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.

Not necessarily. Any measurable amount of fluff will help.

>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.

My point exactly. I say helping reduce the effect of impact "falls" into this
category.

>>If these proto-birds that happen to have feathers bigger than most
>of  their 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.

Bigger feathers _would_ help, IF the animal were arboreal. There _would_ be a
significant difference. But how could feathers help a cursorial form in any
way? They would just get in the way--slow it down. If feathers and other
long, hairy structures were so beneficial to cursorial forms, why don't we
see more of them on ground-dwelling animals? Why do flightless running birds
tend to _lose_ or _reduce_ their flight feathers? Why aren't big cats and
dogs and hyaenas burdened with long hair?

>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 
>whose 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 catching 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.

What other reason? Display? Insect catching? Why not look at it the _right_
way, and understand that feathers evolved for trajectory control, and that
_this_ was the preadaptation for display or insect-catching or whatever. Why
not choose the likelier alternative first? I claim that the catch-all concept
of "display" is a perfect example of the use of a structure that evolved for
something else being suborned for sexual selection, species recognition,
and/or rival intimidation. Thus, I would not expect feathers to have evolved
_as_ display structures, but I would definitely expect feathers, _once they
appeared_, to evolve into different shapes, sizes, and colors, reflecting
(for want of a better term) various intraspecific preferences.

>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 cursorial proto-bird as for an arboreal 
>one.  Besides, Just because whatever selective pressures causing a 
>cursorial 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.

When you find something as compelling as solving the falling problem, or
evading predation, which have obvious and immediate selective advantages,
what reason would you want to look for something farther fetched? Becoming
arboreal is an excellent way of solving the very immediate problem of evading
a predator: just ask any squirrel that my dog has chased into a tree. Leaping
from branch to branch in a tree or between trees likewise helps to solve the
problem of what to do when the predator comes up into the tree after you.
There is clear and obvious selection pressure to evolve better ways of doing
these things.

And if you simply remove the trees and/or remove the predators, as sometimes
happens, you're back down on the ground again, evolving into a
ground-dwelling lineage--a lineage retaining many of the now peculiar and
even useless characters of your arboreal ancestors (like retroverted
halluces).