Richard Wein wrote:
>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.
There is no apparent increase in the number of segments snakes have.
This notion is solely based on the theory that snakes evolved from reptiles
with a lesser number of segments.
>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.
I'm arguing that mutation can't add one new symmetrical segment or
structure to an existing set. This kind of duplication can only occur
through Siamese-twinning of the whole body, and this kind of mutation
has not been viable since the early Cambrian. Atavisms are only
apparent counterexamples, because these are only reappearances
of old segments. In a sense they are not even reappearances; they
may be considered the rare extremes of a species' range of variation.
>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.
I'm unaware of any theoretical causes of Siamese-twinning in human
which are not heritable. It's a morphological phenomenon, and morphology
is heritable.
>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 explicitly describe the mechanisms by which the theoretical progenitor
comes to be and mutates into the variety of Cambrian fauna. I draw diagrams.
What else can I do?
No fossils. I have to agree with Homer Smith that vertebrate origins is
a topic for theorists. Otherwise, reconstruction of evolution is just a matter
of playing with animal figures, arranging them in order to suit yourself.
>You haven't answered my point about how symmetry is *maintained* during
>evolution of code duplicates, without requiring coincidental identical
>mutations in each duplicate.
There is so much symmetry within a vertebrate body, I don't see how you
can entertain the idea that maintaining it requires unacceptable coincidences.
Anyway, your genetic criticism of this morphological theory is underwhelming
because you don't even know whether segments are generated from one
code segment repeatedly processed, or from many separate code segments
being processed.
>>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!
I could try to write like Darwin, always throwing in disingenuous
subjunctive expressions like "...might we not believe...?" but it would
be tiresome for all concerned.
>>>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.
I don't see how Siamese-twinning duplication creates less complex
segments. What would be more complex than multiplying the whole
body?
>>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.
They are added simultaneously.
>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.
A light-sensitive cell is the beginning, the crucial core element of the eye.
But complex eyes have many other parts. It's easier to think of these
parts as being co-opted from neighboring structures than to think of
them arising out of nothing. Muscles near a primitive eye, for example,
could affect direction and focus crudely, and these muscles could gradually
migrate into position to control the eye finely.
>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.
I only argue that complexes of segments came into being in various
patterns and combinations, and that some of these combinations were
successful.
>>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).
The question is, when and how does the duplication occur?
I have all the duplication occurring in the early Cambrian through
Siamese-twinning of entire bodies, followed by reduction winnowing
out unnecessary structures. This conforms to the evidence; it fits the
recognized pattern of reduction and specialization. Your model allows
duplication of specific internal structures without duplication of the
whole organism. A difficult thing theoretically, and not supported by
the evidence.
>You didn't address the point about *fusing* of organisms.
At first the Siamese-twins are independent physiologically; while
enjoying the advantage of greater size and the potential for improved
locomotion, they can fuse at their leisure. I don't see this fusion as
difficult for creatures with identical genomes.
>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.
Various configurations formed, and only a few succeeded. Developing
coordinated movement through gradual evolution doesn't seem problematic.
With these outriggers in place, any kind of fortuitous twitching might move
the creature faster than its competitors.
>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.
You claim they don't correspond to skeletal segments, I claim it's the
best explanation for the pattern. There doesn't have to be a stripe for
every visible segment. Nor must there be a visible segment for every
stripe.
>>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.
Chance is always a factor. And 'embryological mechanisms' doesn't seem
incisive enough in itself.
>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.
Symmetrical striping is a little confusing to the eye, especially among
herd animals. The dazzle-painted effect is a different strategy which I
think is often employed by fishes which have asymmetrical markings.
I don't recall that stripes were much of a factor in pre-radar ship
camouflage; wasn't it just a matter of large odd shapes?
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
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