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