|
Dann Pigdon said (26.6.98):
< I would have to disagree on this point (sorry George!). I think
that
flight most probably evolved quite rapidly. The earliest pterosaur fossils show highly specialised fliers. So too with the earliest bat fossils. There don't seem to be any (AFAIK) supposed "transitionary" forms for bats or pterosaurs. > Yes. Much *adaptive* evolution as compared to drift, is very
fast. Where the "fitness" rewards are high, and adaptation is
merely a question of linear extrapolation, for example making flaps of skin
bigger, making arms longer, enlarging muscles, evolution is in sprint
mode.
Sometimes though, evolution has to pick its way around something. I
believe inventing complex structures (lapsing into anthropomorphy here) is
difficult for evolution. My guess is that making a feather is somehow much
more complex than making a big flap of gliding skin. I think that is one
reason why pterosaurs beat birds into mastery of the air, and why bat evolution
was ten times faster than birds'.
There are two more reasons why bird evolution was so slow. First,
competition with pterosaurs must have been intense. Second, simply because
the cursorial ability was always at a high level in early birds, any adaptation
that compromised it would adversely affect fitness. In bats and
pterosaurs, the ability to run fast was lost so quickly that it ceased to be an
issue.
Back to competition with pterosaurs again, pterosaurs weren't quite in the
same position as other robust (evolutionarilly) groups in being able to slam the
door of the niche on competing groups (I am not advocating group competition
here, only a recurring theme across competitive situations at much lower
levels). Birds just never went away - they were too good at other
things. Like a starfish that just waits for its victim to give up,
gradually exploiting any little opening that appeared, from a position of
external security, birds were able to take the time to evolve their many times
more complex system. They would have wiped out pterosaurs by now even
without the meterorite. (I don't think evolutionary complexity, or indeed
any other kind has been defined, so please don't ask me to justify this!)
Though I think arboreality was a heavy influence on early birds and (?!)
dinos, and that flight evolution is usually fast, in birds it was slow.
< Secondly, I don't think feathered flight was necessarily so unique that it developed just once. Take bats and flying foxes for instance. Two groups that are not all that closely related, but which evolved into similar forms. > As Darren Naish has recently mentioned, the
issues of polyphyly (is that the word?) amongst bats are not settled, and
certainly not in favour of diphyly, more attractive though it undoubtably is,
though not quite as attractive as the idea that we are flightless
bats!
< Just because Archae had feathers and seemed capable of
flight does not mean that it contributed in any way to the line that led to modern birds. > Luckily, since I operate in the realm of personal opinion, I have access to
resources considered invalid in other areas - so I am able to weigh up in my
mind certain probabilities: could parallel evolution have produced two
sorts of feather exactly the same? If they could, would the similarity of
timing make it even less likely? Most importantly, could the two types of
feathered fliers have both survived? Ah - I see you are saying that
Archae's line didn't survive. Errrr . . . I would have to disagreee on
this point - sorry Dann!
John V Jackson
FRKBCFS
|