rst@tardis.UUCP (Robert Thau) (03/25/85)
> A S Romer, Vertebrate Paleontology, 3rd ed, University of Chicago Press, > 1966, pp. 37-38: > > "The record, in fact, fits in better with the opposite assumption: > that the sharks are degenerate rather than primitive in their skeletal > characters; that their evolution has paralleled that of various other > fish types in a trend toward bone reduction; and that their ancestry > is to be sought among primitive bony, jaw-bearing fishes of the > general placoderm type. No well-known placoderms can be identified as > the actual ancestors of the Chondrichthyes, but we have noted that some > of the peculiar petalichthyids appear to show morphologically > intermediate stages in skeletal reduction. Increasing knowledge of > early Devonian placoderms may some day bridge the gap." > [End Romer] > > ... I am sure you would abhor the same failing in > yourself and will take this opportunity to propose to elucidate for > us why the simple sharks should degenerate from more complex > ancestors. This is particularly interesting as it apparently occurred > whie the teleostean fishes were increasing in complexity, and while my > lovely coelacanth ("be still, my heart") was refusing to budge in > complexity. > Glad you asked!!! The problem here is in the meaning of the word "degenerate." The skeleton of the shark is indeed degenerate, in the sense that the closest thing it has to the usual vertebrate bone is its highly modified scales-cum- teeth. The word degenerate, in this sense, is used only to imply that parts of the skeleton have atrophied over time. In other words, the skeleton of the shark is degenerate in the same sense that the tail of human beings (or better yet, our gill slits) are degenerate. However, the sharks themselves are not "degenerate" organisms. Evolution is not a one-way graded progression toward some goal, along which sharks can be said to have slid backwards. The lack of a skeleton and a swimbladder in sharks is part of a whole suite of adaptations for quick-striking predation. Over the course of time, the ability to swim quickly and strike rapidly has proven more important to the survival of sharks than the protection of a bony skeleton, so the excess weight has been dropped. The coelecanth's persistence is entirely consistent with punctuated equilibrium; one can simply say that it inhabits a niche which has stayed relatively unchanged for the past however-many-million years, and that the population is large enough to prevent substantial genetic drift. (Of course, it helps to back this up with enough info on the lifestyle of coelecanths to show that the niche at least might be as old as the fish. I should, but I don't know much at all about coelecanths). I also don't know enough about evolutionary theory to give an equivalent argument in gradualist terms; any takers? Robert Thau rst@tardis.ARPA
ethan@utastro.UUCP (Ethan Vishniac) (03/26/85)
> > The coelecanth's persistence is entirely consistent with punctuated > equilibrium; one can simply say that it inhabits a niche which has stayed > relatively unchanged for the past however-many-million years, and that the > population is large enough to prevent substantial genetic drift. (Of > course, it helps to back this up with enough info on the lifestyle of > coelecanths to show that the niche at least might be as old as the fish. > I should, but I don't know much at all about coelecanths). I also don't > know enough about evolutionary theory to give an equivalent argument in > gradualist terms; any takers? > > Robert Thau As I understand it the difference between gradualism and punctuated equilibrium models has to do with the pacing of evolution. Both schools would agree that a stationary isolated ecological niche inhabited by a well-adapted organism would not give the opportunity for morphological change. Biochemical change would occur, but would not show up in the fossil record. "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.*