Re: Is evolution really the central theory for all of biology?

From: David C Campbell <amblema@bama.ua.edu>
Date: Mon Sep 19 2005 - 15:01:56 EDT

>Indeed, even as mere circumstantial evidence the pseudogene data become
ad hoc and scientifically useless, since under common descent there
must be convergent mutations to explain common nucleotides that must
have arisen independently (eg, in the urate oxidase pseudogene). If
mutations can be viewed as independent where common ancestry cannot be
the explanation, the why can't all mutations be viewed that way? From a
scientific perspective, it is special pleading to claim these are
evidence for evolution.<

Convergent evolution is strongly expected if evolution happens at all.
A classic example comes from the similarly streamlined bodies of
squids, active sharks, tuna, ichthyosaurs, and dolphins. All encounter
the same basic hydrodynamic forces and the same sort of body shape
works well for all of them.

DNA adds another source of convergence. In addition to the good
possibility that a similar DNA sequence will be useful to different
organisms with similar niches, there is the 1/4 chance of random
similarity between two DNA strands (because A, C, G, and T are your
only options, unless you count insertions/deletions as a fifth option).
 How do you distinguish between convergence and actual evolutionary
heritage? A couple of factors help.
Is there any evident reason for convergence? For example, the proposed
whale-hippo link, based on molecular evidence, has been questioned
because the common physiological demands of spending extended periods
submerged might put similar demands on various molecular functions.
(Overall, the data do support whales not being far from hippos, but I'm
not sure if there is a consensus yet on just how close.) On the other
hand, every known organism needs functional ribosomes to survive, so
ribosomal DNA sequences do not have an obvious reason to be evolving
convergently, apart from the random similarities.
What does the majority of the evidence indicate? A problem here is
sorting out linked characters. E.g., a pearly shell and weakly
developed hinge teeth correlate in bivalves because pearly shells are
more flexible and don't have to align as carefully as more rigid
shells. Well-developed hinge teeth help align the shell precisely.
Is there other evidence of convergence? E.g., pearly shells evolve to
other types repeatedly in the mollusks, so in a poorly studied group I
would be cautious about relying heavily on pearly versus non pearly.

The laws of physics and chemistry dictate that certain body plans,
biochemical approaches, etc. work better than others. Evolution, being
constrained by these, will tend to find similar solutions. Also, the
evolutionary heritage of an organism may provide certain constraints or
a relatively easy starting point for a particular goal. Bivalves have
specialized as headless filter feeders and have greatly diversified
witihin the constraints of that starting point, so convergence in
various aspects is fairly common.

In short, convergence is not a valid challenge to the legitimacy of
evolutionary explanations, though it is a challenge to the correctness
of specific models for particular groups.

----------------------------------------
Dr. David Campbell
425 Scientific Collections
University of Alabama, Box 870345
Tuscaloosa AL 35487
"James gave the huffle of a snail in
danger But no one heard him at all" A.
A. Milne
Received on Mon Sep 19 15:05:14 2005

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