From: Cliff Lundberg <cliff@cab.com>
>You can't deny the tide because you can theorize that there is a
>little counter-eddy among the rocks.
I don't deny the tide. I just don't accept without evidence that there can
be no exceptions, especially when we apparently do see exceptions, such as
the increase in segments in snakes. Of course, if you dismiss all the
exceptions as only apparent, then you won't see any exceptions. But this is
circular reasoning.
>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.
This makes it sound as if I'm arguing for a mutation to create a different
functional structure. I'm only arguing that it can duplicate an existing
one.
>>>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.
So you keep saying. I don't agree.
>>Are you saying that genetic birth defects are the same thing as
>>Siamese-twinning?
>
>Siamese-twinning is a birth defect to us.
You're still evading my question of whether we ever see heritable
Siamese-twinning in humans. If not, then your claim that we see this
mechanism in humans is false.
>>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.
A paleontologist can study fossilized phenotypes. But you have no fossils of
your proposed organisms. If you can't propose a mechanism by which these
organisms could have evolved, then why should anyone believe that they
existed?
>>>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?
Where paired appendages are similar but not identical, I think both
appendages are generated from the same piece of basic code, but there is
*additional* code for the distinctive features.
>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.
You haven't answered my point about how symmetry is *maintained* during
evolution of code duplicates, without requiring coincidental identical
mutations in each duplicate.
>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.
I wish you wouldn't keep stating your hypotheses as if they were facts!
>>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.
In other words, it's duplication of less complex segments, as I said.
>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.
Your model involves partial duplications too, unless your 4 proto-limbs were
all added simultaneously.
>>>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.
So you think the eye evolved out of another organ entirely? Is that really
any easier to believe than that it evolved from scratch? I find it more
difficult to believe. As I pointed out before, Dawkins and others have given
a scenario for gradual evolution of an eye from a light sensitive cell. I
haven't seen any such scenario for evolution of an eye from another organ,
and it's hard to conceive of one.
>>>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're still ignoring the problem that natural selection cannot look ahead
to what will be advantageous later. Your crescent shaped segments must have
an immediate advantage if they are to be selected.
>>>>- 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?
Duplication by mutation (at least that's *my* answer--I haven't read
Dawkins' view on organs, but I guess it would be the same).
>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 address the point about *fusing* of organisms.
>>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.
You still don't explain how proto-limbs with no connecting muscles or nerves
could give an advantage in locomotion. In fact, one would expect them to be
highly disadvantageous since their "oars" are all pointing in different
directions.
>>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.
I don't think I can comment any more on this without seeing the sources for
myself.
>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.
I think the similarity is extremely poor. There are stripes which merge and
stripes on the posterior which don't correspond to skeletal segments. And
the number of stripes is far less than the number of segments in your
proposed proto-vertebrate.
>>>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.
The divergences aren't specified in terms of their function. They could be
considerably different and still be as effective. So I suggest that the
details of the pattern are a matter of chance and/or a consequence of the
embryological mechanisms.
>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.
Dawkinsian evolution is constrained by genetic and embryological mechanisms.
Adapatations are not always optimal. Anyway, I doubt whether the zebra's
stripes have evolved as grass-like camouflage. I can't imagine them being
very effective for that purpose. They're more like the sort of dazzle
patterns used on ships in World War 1, to break up the outline of the ship
and confuse the enemy. For this purpose, it's best that the stripes not be
too regular, and if you look at pictures of dazzle-painted ships, you'll see
that the stripes don't all go the same way.
Richard Wein (Tich)
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