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Re: BCF& COMMENTS ON IT (really long)



George Olshevsky writes:

>Well, as far as I can tell, evidence from physics truly >falsifies< the idea
>that birds evolved the ability to fly from the ground up. As far as I'm
>concerned, it is 100%, no question disproved, even if a few diehards haven't
>yet come around to thinking so.

Surely this depends on what model of ground-up you are talking about.  The
"running-along-flapping" model, perhaps; but fluttering (as I have suggested,
though I wouldn't go so far as to call it a hypothesis), leaping from rock to
rock, etc., perhaps not.

Therefore, at some point in the evolution of
>avian flight, well before the appearance of _Archaeopteryx_, birds >must<
have
>gone up into the trees. So any ground-dwelling, cursorial descendants of
those
>flying, arboreal pre-archaeopterygids would, by my definition, be
>>secondarily< flightless. 

Arboreal, perhaps; but why flying?  There are lots of arboreal animals that
don't fly.  Secondarily terrestrial, perhaps?

>The dictionary defines "flight" as "passage through the
>air," so if leaping from branch to branch, falling, parachuting, and gliding
>were among the features of an arboreal lifestyle in pre-archaeopterygid
>dinobirds, then they were technically fliers in my book, and their
descendants
>were secondarily flightless.

By this definition, all squirrels fly.  So do tree kangaroos, anolis lizards,
and Tarzan.   So does a kid falling out of a tree house.  This is so far from
what most people understand flight to be that I think it prevents people from
understanding that your hypothesis may be a lot closer to the mainstream view
than you make it appear.

>_Archaeopteryx_ and birds arose from >something<, and in BCF the same
>something (or somethings) also gave rise to theropod dinosaurs.

I think you have just reduced your own hypothesis to an identity.  Obviously
Archaeopteryx and theropods have a common ancestor.  They would have a common
ancestor if birds had arisen directly from frogs or dragonflies; so all the
statement above says is that evolution happens and all life on earth evolved
from a single ancestor.  I know this isn't what you mean, but I suggest that
you have to define your hypothesis more rigorously - even (shudder!)
cladistically.

> Certainly an opposable hallux occurs in many, many modern arboreal
>birds; this alone speaks volumes about the "usefulness" of the structure.

For perching, certainly.  In another post I have given examples of climbers
relying on the hallux, but all these are derived from perchers.  I would say
that any climber that must hold its body away from the trunk of a tree
needs an
attachment point above and below the centre of gravity to do so - that is, an
upper suspensor and a lower brace.  In living climbing birds the upper
suspensor is usually the anterior-facing claws, though in hoatzin chicks it is
of course the wing claws and in many parrots it is the bill.  The brace is
either the hallux or the tail, except in the cases I have just mentioned (and
even in parrots the most specialized trunk climbers, the pygmy parrots of the
genus Micropsitta, have a woodpecker-like spiny tail used as a brace.)  The
hallux is not important, or as important, in tail-braced climbers (which
include, besides woodpeckers and pygmy parrots, the certhiine treecreepers and
the dendrocolaptine woodcreepers (plus a few others) among the passerines.

>BCF does not assert that manual digit loss is a climbing adaptation. Quite
the
>contrary! BCF states that manual digital loss is extremely unlikely in
>climbing animals, for >exactly the reasons you delineate!<. They need to have
>as many fingers as possible. 

Note, however, that some climbers, like chamaeleons and koalas, have actually
reduced the effective number of forelimb digits through fusion (real or
functional) to produce a grasping "pincer", that gibbons have greatly reduced
the thumb to produce a brachiating "hook" and that sloths have reduced digits
even further for the same purpose.  Does any theropod with reduced digits show
patterns similar to these?

The BADD model
>>cannot< account for this exact pattern of manual digital loss in ground-
>dwelling cursorial bipeds, which presumably would find retaining all five
>digits more "useful" for grasping and holding prey (since they were not
>converting their forelimbs into wings). BADD gives >no functional reason< for
>the loss of manual digits IV and V; this is just >something that
happened,< ad
>hoc, that turned out to be "useful" when the forelimbs were suddenly exapted
>for flight.

Another one of my earlier suggestions was that the forelimbs could have been
used in concert, like ice tongs, in which case there might have been little
need for the outermost (and posteriormost) digits.  If this is true it could
explain digit loss in terrestrial species; anyway, I think it shows that you
can come up with an explanation for digit loss that fits the ground-up model.

Digital loss, whether manual or
>pedal, is >not< something that occurs with ease in tetrapods; consider
that it
>took most of the Cenozoic Era for horses to change their five-digit feet into
>one-digit feet. 

This is because, perhaps, the outer and inner toes served a bracing function
for the foot in running that is now taken over by a tendon arrangement - I
don't recall the exact details, but they are believed to have formed part of
the functional running mechanism even if they didn't touch the ground. 
Certainly in birds digit loss appears to happen fairly easily; among
woodpeckers there is more than one case of three-toed and four-toed species in
the same genus.

If you want to have a whale evolve from
>mesonychid carnivores, you put them in the water >first<; they don't evolve
>flippers and >then< go into the water! Likewise with birds: Put them in the
>air, or at least, up in the trees, >first<. Then let them evolve all their
>adaptations for flying. Why is this not perfectly obvious??

However, it is now believed that tetrapod limbs evolved before tetrapods
crawled out onto the land; the question is whether the structure concerned is
advantageous to the animal where it is, not its degree of complexity.

>This is the same kind of miracle, though actually far less likely
>(because there are lots more molecules in a cup of water than anatomical
>features in a tetrapod skeleton), as the kind BADD wants us to accept: that
>twenty or thirty anatomical features, unrelated to flying, accumulate in a
>single animal ("align themselves") and thus allow it to fly. I am not calling
>exaptations miracles; I am calling the serial alignment of twenty or thirty
>unrelated exaptations a miracle.

It is the words "unrelated to flying" that trip you up here.  An adaptive
suite
can consist of many seemingly dissociated structures that nonetheless form a
functional complex.  Your serial alignment is only a miracle if these
characters did not form part of a functional suite adapted to something the
animal was doing short of flight; if they did, there is nothing miraculous
about it.

If paedomorphosis were the >exclusive< means of acquiring
>flightlessness, then ratite birds would >exactly< resemble giant chicks--and
>of course they do not. Ratite hind limbs are certainly not paedomorphic, for
>example...

Storrs Olson argues that in many ways the flightless ratites DO resemble large
chicks, in feather structure, limb proportions etc.  Alan Feduccia, in his
article on flightlessness in "A Dictionary of Birds" (Poyser 1985), states
that
"Whatever the relationships of the various ratite groups.. they show similar
neotenic features to those found in other flightless birds."

Like Matt, I am not trying to "disprove" BCF - only to suggest that many of
the
things you argue that only BCF can explain are at least potentially compatible
with other hypotheses, so that BCF cannot be accepted "by default" as the only
explanation that fits all the facts. 
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net