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
