Richard Wein wrote:
>The absence of new segments is not a known fact--it's part of your
>hypothesis.
It's an inference from the pattern of reduction and specialization of
segments, a venerable generalization.
>>>You yourself provided the counter-example of additional segments in
>snakes.
>>
>>I meant to offer this as an example of a traditional idea which is
>erroneous.
>
>You *claim* that it's erroneous. But we don't know that.
I claim that it's an idea that's in conflict with the pattern of reduction
and specialization of segments. The burden of proof should fall upon
those whose claims are anomalous, not on those who accept an
established pattern.
>A trend is just that. It may have exceptions.
You can't deny the tide because you can theorize that there is a
little counter-eddy among the rocks. A trend is something whose
causes need to be understood. Gradual loss and distortion of segments
is perfectly understandable under RM&NS. Insertion of new segments
is unlikely to the point of virtual impossibility, unless you invoke
a mysterious 'control gene' which has the power to create new
functional complex structures.
>>Polydactyly is an atavism, as are extra legs in insects.
>
>Again, this is your *claim*. As I said before, your claims on this subject
>*may* turn out to be true. But I see no reason to accept them in the absence
>of evidence.
A mutation that creates a completely new digit, in the right place, with all
the
needed infrastructure, is impossible. The skeleton is a limited expression of
a primordial structure which was more complex (and more symmetrical).
Post-Cambrian evolution is a matter of harnessing the expression of this
structure, limiting and distorting the phenotypic expression of the structure.
Atavisms are hitches in the process. I suspect that atavisms play no
significant
role in evolution. They may be considered rarely-seen examples of the extremes
of variation (in number of segments) in the phenotype.
>>Unfortunately, there are genes for birth defects.
>
>Are you saying that genetic birth defects are the same thing as
>Siamese-twinning?
Siamese-twinning is a birth defect to us. But in the early Cambrian
it was a dynamic evolutionary mechanism. I would think that it could
be caused in various ways, some of which are heritable.
>But what is the use of a theory that considers only morphology and ignores
>the issues of genetics and selection, which are vital mechanisms of
>evolution?
Genetics is about vital mechanisms which were always there, which do
not in themselves evolve. To study evolution is to study phenotypes.
>>I ascribe the symmetry first to the original morphological duplication; a
>>pair
>>of twins is symmetrical. As for the symmetry of the subsequent distortions,
>>I presume selection favors symmetry along some morphological axes. Oars,
>>for example, should be the same size.
>
>I can't accept this. We're talking about more than just size. A functional
>oar requires structure: shape, muscles, nerves, etc. For a pair of
>appendages having seperate genetic code to remain symmetrical as they
>evolved would have required an unacceptable degree of coincidence,
>selection not withstanding.
The evidence shows that paired appendages do in fact maintain symmetry.
But if this is a result of their being built from the identical code, how do
you
explain the exceptions, like the fiddler crab or the reproductive limb of an
octopus? In my model the symmetry of paired appendages stems from
their being primordially identical duplicated structures. The expression
of these structures can be limited in different ways, causing asymmetry,
but this is seldom selected for. This phenomenon can be viewed as the
action of genes which regulate the developmental process. So, do I
admit 'control genes'? Yes, but when they cause mutations which
apparently have more segments than their parents, they are not
creating anything new; these are atavisms.
>As I say above, your model also requires Dawkinsian duplication of
>segments. The difference is just that it requires duplication of less
>complex segments than the conventional model.
A further bigger difference is that my model has duplication of
segments taking place only during a brief early Cambrian heyday,
when crude Siamese-twinning mutations had a chance at survival,
before well-evolved metazoans were around to outcompete such
experiments. Actually, the duplication of the whole body is duplication
of a more complex 'segment' than a smaller, internal duplication of
a structure. But it's a much easier thing to accomplish.
>>The forward-sweeping radials are beginning to take on sensory and
>manipulative
>>roles, 'on their way' to fusing their many parts into jaws and skulls.
>
>*Beginning* to take on sensory roles? I thought you didn't accept the
>existence of incipient organs! ;-)
When you have a lot of parts to play around with, some of them can
gradually take on new functions. No problem of incipience there.
When you don't have this plentiful stock of parts lying around, you
do have an incipience problem with the beginning stages of a new
structure.
>>As to the former bits, those are totally speculative; one of them was
>>inspired by the back-swimmer beetle. They're just small formations
>>of segments that might be advantageous in some biomechanical way
>>over other combinations.
>Not a very convincing argument! The analogy with a beetle is irrelevant. The
>beetle is an integrated organism, with articulated parts. Your organism is
>just a load of segments stuck together, with little or no integration. (The
>more integration you assume, the more complex and Dawkinsian becomes
the subsequent duplication of segments.)
Siamese-twinning, the simple agglutination of whole bodies, is not a
complex mechanism. What follows is gradual Dawkinsian Darwinian
evolution, adapting the mutation to take advantage of its new complexity;
but this kind of evolution does not duplicate any segments, it just distorts
the parts that are already there.
>>>- You write: "Circulatory and nervous systems fuse and communicate." This
>>>seems like a major, and difficult, step, which is unnecessary under the
>>>Dawkinsian model. You also have other organs fusing to form, for example,
>>>the multi-chambered heart. But fusing of organs in this way seems to me
>>more problematic than the Dawkinsian duplication that you reject.
Well, it does get around the problem of incipience. How does conventional
theory explain the multi-chambered heart, and other lobed and paired
organs? Why start up a brand new heart chamber, with all the difficulties of
incipience, rather than improving the existing chambers? Oh right, the
control gene. Is there anything the control gene can't do?
>Have you thought about the genetic changes required to accomplish fusing of
>organs? I say again: evolutionary fusing seems just as problematic to me (if
>not more so) as the Dawkinsian duplication that you reject. Are there any
>examples of evolutionary fusing of organs?
Many primitive segmented metazoans have nearly full complements of
organs in each segment. Just about any organ in a highly evolved metazoan
could be seen as an enlargement of such a primitive organ. Most of the
duplicated organs disappear, but one or a few remain and serve the
whole body, not just one segment.
>You didn't answer my question. What use were the "limbs" before they evolved
>any musculature and nerves linked to the main trunk? If they were no use,
>then this arrangement would not have been selected over a more random
>arrangement.
First, there is the advantage of sheer size. This morphological multiplication
is a way to increase size without making all the adjustments necessary to
simply enlarge an organism as it is. Out of many such complexes, some
fortuitously had a little advantage in locomotion. These succeed and
refine the advantage gradually.
>>Right, in principle there is little information involved. But selecting
>>blotches through RM&NS is not going to do it; that's working on separate
>>pixels, when what you want is vector graphics and repetition.
>You're continuing to attack the straw man that I've already rejected. I
>don't believe that blotches evolved pixel-by-pixel into stripes.
When I say "Right', that means I agree with you. I was just reiterating.
>It's not clear to me how much of this is from the sources you cited, and how
>much is your own extrapolation. I also note that your sources are very
>old--1967 is ancient in this field.
It's not at all clear to me what the evolutionary ideas of the cited
developmental
biologists may be, or whether they have any evolutionary ideas at all. It's
just
something I see as supportive of my evolutionary model. You can't dismiss
something just because of the date, you have to make some allusion to why
it's outdated. You see the similarity between zebra-stripes and my prototype
vertebrate skeleton as coincidence. I don't, I think the similarity is great
enough to require an explanation.
>>Longitudinal stripes are a lot simpler than zebra striping patterns.
>
>I disagree. Where the zebra pattern diverges from straight stripes, the
>divergence is pseudo-random. It could be considerably different and still be
>just as effective (i.e. the pattern is not highly "specified"). And earlier
>you cited the crispness of the zebra's stripes in support of your position.
>But the common garter snake has even crisper stripes than the zebra.
Crisper because simpler. The divergences from straight stripes in the
zebra are quite specific. In the head, the stripes angle forward in the
same way in every zebra, but the point at which this begins can vary.
In the legs, you have transverse stripes, quite distinct from the stripes
which are transverse on the axial skeleton. I don't see what camouflage
advantage this would offer. Simple Dawkinsian evolution would have
the same vertical grasslike striping evolving over the whole body.
If the stripes tie in to the skeleton, in which axial and appendicular
structures are distinct, this phenomenon is explained.
>You would say that, wouldn't you? But I see no reason why the switch from
>lateral to longitudinal stripes would be any easier than the evolution of
>longitudinal stripes from scratch. If anything, it would probably be more
>difficult.
I see the evolution of anything from scratch as difficult. It's easier to
envision existing elements being distorted as they adapt to new uses.
>>but they may be simple enough to have evolved in other ways.
>
>In that case, why should we assume that lateral stripes switched to
>longitudinal stripes, and not the other way around?
Lateral stripes and fewer and simpler. Reduction and distortion of
existing elements is easier than elaborating brand new elements
out of nothing.
>I'm afraid that these arguments of yours are starting to take on a distinct
>air of desperation!
That is not much of a scientific argument, but if that's all you have,
then go with it.
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
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