Oct2893 07:28AM Denton's Criticisms A little while ago, I agreed to take a look at Denton'

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Oct-28-93 07:28AM Denton's Criticisms Organization: Rutgers Univ., New Brunswick, N.J. From: trott@gandalf.rutgers.edu (Richard Trott) Message-ID: Newsgroups: talk.origins A little while ago, I agreed to take a look at Denton's criticisms of the molecular clock starting on page 294 (chapter 12) of his _Evolution: A Theory In Crisis_. Here are my comments, but they are largely from the perspective of a somewhat informed layperson. I would appreciate it if anyone could add to or criticize what I have to say. Argument => page 296 == "Molecular clock is a tautology." This is simply not the case. One does *not* have to assume evolution. The molecular clock is an opportunity to falsify evolution. If there was, for example, no molecular difference between proteins in humans and mosquitos, but a difference between proteins in humans and bears, there would be a problem for evolution. Denton's logic on this issue is completely unacceptable. Using it, one comes to the conclusion that the revolution of planets is a tautology that assumes gravity. Denton next asks what possible mutational mechanism could account for the different speeds of change in different proteins. Of course, a mutational mechanism is not required, merely a selection mechanism. (Any highly informed individual want to answer Denton's stuff about drift? As far as I know, drift has nothing to do with it so the criticisms are irrelevant, but I could be wrong.) When Denton addresses selection on page 300, he claims that it too is a tautology. However, selection pressure on a protein ("functional constraints" in Denton's words) is not tautological for the same reason the molecular clock is not. If one could not find the results of selection pressure (such as the molecular clock), then there is no evidence for natural selection. This is not tautology, it's science. Denton writes: "Although it is put forward as a solution to the problem of different rates of protein evolution in different families of molecules, the only evidence for the hypothesis is the observation it claims to explain." One could change "different rates of protein evolution in different families of molecules" to "the movement of the planets around the sun" and one has an equivalent statement about gravity. Quite simply, different proteins changing at different speeds is a logical conclusion of natural selection. Denton, on page 300, commits a very noticeable logical error: "Just because some vertebrate haemoglobins such as carp and man differ from one another at up to eighty amino acid sites -- while their histones are identical, it cannot be inferred from this that the histones are under more stringent selective constraints." Histones are essential to the genetic mechanism. The genes that code for histones show differences between organisms in areas that don't code for the histone. On the other hand, the specific sequences that code for the histones show little or no change. The change in the "silent" areas is, of course, expected from drift. (This, of course, has nothing to do with Denton's discussion of drift earlier because we are not trying to explain different rates of change for different proteins, merely the fact of difference in "silent areas.") Haemoglobin, on the other hand, has high selection pressure, as it is essential to life, but the extreme selection pressure of histones, which are essential to life *and* replication. (One can get by with haemoglobing that's "good enough," but even a slightly less than perfectly functioning histones has enormous consequences for every aspect of the organisms function, if the organism ever manages to form in the first place.) Deonton concludes: "Similarly, we do not conclude that the selective pressures on vertebrate limbs are any less intensive than those on vertebrate spinal columns merely because the former exhibit much greater interspecies diversity than the latter." But this is simply 100% wrong! We *do* predict that selection pressure on limbs will be less than that on spinal columns! And we *do* conclude that the evidence bears this out! Denton continues with the following (p. 300): "Moreover, there is not a scrap of empirical evidence to suggest that there is any systematic difference in the tolerance of different functional proteins to mutational change." Is there any evidence to suggest that the tolerance is the same or that it is not systematic? Why would we expect it to be the same or to not be systematically different? Besides, Denton may very well be overstating the case. The quotation that Denton supplies to back up his statement simply talks about why there was no laboratory experiment done (as of 1977) determining sensitivity to amino acid substitution of histones. Is it logical to conclude from this that "there is not a scrap of empirical evidence" that there are systematic differences between *any* proteins? (Does anyone more knowledgable than I know of any evidence?) Denton goes on to argue against the possibility of a protein changing because of the selective pressure on the protein. Denton's arguments are true, but only for the active site of critical and sensitive proteins (like histones which, lo and behold, exhibit little change!) and for mutations that would significantly change the shape of a protein. So what? Are these the only types of mutations? Is Denton trying to say that these types of mutations are *never* selected for, or just extremely rarely? If the former, does that really make sense? If the latter, who cares? Denton concludes, on page 301: "Again, it is the sheer universality of the phenomenon -- the necessity to believe that the functional constraints in *all* the members of a particular protein family, say A, in *all* diverse phylogenetic lines for *all* of hundreds of millions of years have remained precisely five times as stringent as those operatind on the members of another protein family, say B -- which fatally weakens the theory." But isn't this highly illogical? Isn't the "necessity to believe" really just drawing the logical conclusion that certain protein families have higher selection pressure than others and that the degree to which these selection pressures are greater *when averaged over the long haul* (not precisely at all times, as Denton erroneously concludes) would be fairly (not precisely) constant? Denton then goes into his well-known misrepresentation of evolutionary trees based on molecular evidence. He points out that the lungfish is equidistant (roughly) from the lamprey, other fish, mammals, etc. This is, of course, hardly evidence undermining evolution because the lungfish used is not the ancestor of these other creatures, it is a modern species. The term "living fossil" is, of course, just a euphemism to talk about how little it has changed morphologically over the millions of years. Of course, pressure on morphology can vary from species to species and from environment to environment much more easily than, say, selective pressure on histones, so one should not be surprised by the variable morphological evolutionary rates of different species. -- Rich Trott trott@gandalf.rutgers.edu


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