Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site utastro.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!genrad!panda!talcott!harvard!seismo!ut-sally!utastro!ethan From: ethan@utastro.UUCP (Ethan Vishniac) Newsgroups: net.origins Subject: Re: Horsefeathers Message-ID: <1163@utastro.UUCP> Date: Tue, 26-Mar-85 11:38:09 EST Article-I.D.: utastro.1163 Posted: Tue Mar 26 11:38:09 1985 Date-Received: Fri, 29-Mar-85 03:19:03 EST References: <827@uwmacc.UUCP> Distribution: net Organization: U. Texas, Astronomy, Austin, TX Lines: 58 > > > You want maybe some sort of useless > > half-feather? Why would something like that be selected for? Why would > > you expect a specie with useless features to survive long enough to leave > > any kind of fossil record? > > Like the Irish Elk? > > I assume from what you say that we *wouldn't* expect intermediate > features, that we must go from one developed structure to another > - no incipient features. Would we, then, expect instantaneous > appearance of fully developed feathers? Anything less would not be > selected for? Or what? > -- > Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+-- It is an interesting problem to speculate on the origin of highly specialized structures. Indeed, evolutionary theory would lead us to expect that they should have developed from other structures that conferred some definite advantage to the organism that posessed them. From this point of view the features that allow birds to fly are most fascinating. At first glance nothing would appear more useless than a partially developed bird. However, upon further examination we see that the issues involved are more subtle. Consider the feathers of that strange beast. There is nothing bizarre about a non-flying creature having feathers. They are a reasonably good form of insulation. This tells us, incidentally, that Archeopteryx was probably warm-blooded (insulation is not a good idea for a creature that needs exposure to the sun to warm up). What is interesting about Archeopteryx is that it has feathers that appear to be structurally modified for flight. In other respects, Archeopteryx is not well adapted for flight. Its lack of a large breastbone and the unfused nature of its skull and backbone show that it had not yet minimized its body weight and adapted for powered flight. It could have been an adequate glider. The evidence from its claws that it was a tree climber is particularly suggestive in this respect. What does all of this suggest? On evolutionary grounds we would expect that there was a preceding species with feathers for insulation, but without the wing feathers (I forget the technical term) necessary for flight. Although such feathers are not well preserved by fossilization we do expect them to turn up eventually (even the "hair" on pterosaurs has been preserved in some specimens). Could it have coexisted with true birds? Only if it could sucessfully compete within some ecological niche. Since there are tree-climbing gliders around today this doesn't seem improbable. The peculiar combination of features seen in it indicate that if it was not a direct ancestor of modern birds, it was still closely related to such an ancestor. I note in passing that there is no firm evidence that it did coexist for any length of time with essentially modern birds. The point of the above is that there is nothing about the development of flight that requires that all of the necessary adaptations be made at once. No species represents a truly "perfect" adaptation. Archeopteryx, like others, was adequate, given its resources and its competition. "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan *Anyone who wants to claim these opinions is welcome to them.*