From: Cliff Lundberg <cliff@cab.com>
>Some of the advantages are:
>--The Cambrian explosion is explained; a mechanism for rapid formation
>of complex vertebrates and arthropods is outlined.
But that begs the question (of whether your mechanism is an improvement on
the conventional one). I see no reason why evolution by your mechanism
should be any more rapid.
>--The nature of post-Cambrian evolution (reduction and specialization
>of segments, no new body plans, no rapid evolution) is explained.
But this description of post-Cambrian evolution is debatable. And, as I've
said before, even if it's true, it says nothing about the mechanism of
Cambrian/pre-Cambrian evolution, which, according to you, is where all the
novel parts arose.
>--The evolution of the vertebrate form and its components is explained.
>--Solutions for specific problems follow from the model, ranging from
>lines of flexure in embryonic skin to zebra stripes to the evolution of
jaws.
Again, this begs the question of whether your explanations are any better
than the conventional ones. I think they're a lot worse!
>There are lots of criteria by which an argument can be judged, but I don't
see
>why being ad hoc is a problem.
Because *anything* can be explained by a sufficiently ad hoc explanation, no
matter how wrong it is. But I suppose this is of a piece with your rejection
of parsimony as a criterion in model selection. In rejecting parsimony as a
relevant criterion, you're rejecting the prevailing scientific method.
>You have not furnished counter-examples to the general claim that new
>segments have not been added since the Cambrian.
You yourself provided the counter-example of additional segments in snakes.
I provided the examples of extra legs in fruit flies and extra toes in
mammals. You rejected the latter as being the reactivation of atavistic
genes, but offered no evidence to support that argument. I don't rule out
the possibility that you're right about this. I just see no good reason to
accept it in the absence of evidence.
>I remind you that this
>is not my invention; the principle of reduction and specialization within
>sets of homologous structures is an old one. Gould has commented on
>it without contesting its truth.
I have no problem with reduction and specialization, I just don't think
they're the *only* processes operating. Does Gould agree with your claim
that new segments have not been added since the Cambrian? (Even if he does,
that doesn't necessarily make it so, but I will be more
impressed.)
>>But let's suppose you're correct that no new segments have been formed
>>during post-Cambrian evolution. The question still remains: why do you
>>prefer Siamese twinning to duplication of segments by the Dawkinsian model
>>for *pre-Cambrian* evolution?
>
>It's just logically and mechanically much more feasible. Siamese-twinning
>is understandable, demonstrable, observable, even in our own species.
>We can see how it happens. But a mutation that inserts a new segment
>within, e.g., the vertebral column, producing a higher number of segments
>than had ever been known in that species' range of variation, that seems
>a hard thing to do. Unless you invoke the deus ex machina of a "control
>gene" that can do anything.
I don't say it can do *anything*. But duplication of an existing part is not
far-fetched. And why should such a mutation be restricted by the number of
segments that have been present in the past?
>>But Siamese twinning isn't heritable (is it?). Genetic duplication (by the
>>Dawkinsian model) is a better explanation precisely because it's
heritable.
>>Perhaps it's much less likely to occur in the first place, but, once it
does
>>occur, the change will be retained.
>
>In my scheme Siamese-twinning is heritable.
First of all, is such a heritable Siamese-twinning ever seen in humans? If
not, your argument that this mechanism is observable in our own species is
invalid.
Reading your article again, I see you say : "Some define "parabiosis" as the
artificial joining of embryos; herein the term refers to embryos naturally
conjoined due to a heritable mutation." But you fail to mention what sort of
mutation you have in mind. (BTW this lack of discussion of genetic
mechanisms is noticeable throughout your article.)
It seems there are two main possibilities, which I'll call MC (multiple
copies) and SCCC (single copy + control code). MC involves duplicating the
genome for an entire segment, so an N-segment organism will have N copies of
the genome. SCCC means having only one copy of the genome for a segment,
plus some control code to cause the genome to be expressed N times.
MC is simpler in the short run, but you can't get very far like that. On
your page "Vertebrate and Arthropod Progenitors; Their Form and Origin", you
write: "The segmented biserial radials come to serve as rowing organs; some
of them, in some lineages, evolve into legs and pincers." SCCC is essential
by this time, otherwise there could be no symmetry of limbs. Symmetry under
MC would require identical independent mutations in each limb. Given the
obvious difficulty of switching from MC to SCCC once you get started, I
would suggest that vertebrate evolution had to await the rise of an SCCC
lineage.
Of course, addition of extra segments under SCCC is evolution by the
"Dawkinsian" model! So I think the real difference between us is not over
whether the Dawkinsian model is ever valid, but over how complex an organism
can grow and still evolve by this model. Of course, it's easier to accept
that this can work a for simple organism than for more complex ones, which
would explain why it's so rare in the post-Cambrian. But you haven't given
any grounds for establishing any particular complexity barrier beyond which
it can't operate. On the other hand, restricting it to only the simplest
organisms, as you do, leads you into other problems...
- You show a hypothetical step-by-step formation of the protovertebrate.
But it's not clear that the intermediate steps have any selective advantage.
I can accept that size is an advantage. But why do your three-pronged forms
have an advantage over simple linear or four-pronged forms? What advantage
does a crescent with a little bar in the middle have over other
three-pronged forms? (In fact, it's almost a reversion to a linear form.)
What advantage does the final form in Figure 1 gain from having some
of its radials curved forwards?
- 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.
- You then proceed to duplicate the entire organism to produce some
proto-limbs (Fig. 2). Again, what selective advantage does this new
variation have? There is no musculature in place yet to operate these limbs.
Neither is there a central nervous system to control them. The circulatory
and nervous systems of the limbs have not yet fused with those of the main
body. (If you propose that the nervous systems are fused by the same
mutation that created the limbs, then this is a complex Dawkinsian operation
of the sort you reject. In fact it's worse, because Dawkinsian operations
generally add parts in the same relation to existing parts, such as an extra
vertebra, while you're adding parts in a completely new relation.)
Sorry, but it seems to me that you're just sticking parts together to get
the shape you're aiming for, without thinking about the genetic mechanisms
and selective pressures that are required.
>>Evolutionary biologists today no longer propose the kind of gradualism
that
>>you keep attacking (if they ever did).
>
>AFAIK it is generally believed that segmented organisms were and can be
>gradually elaborated through evolution, despite the establishment of the
>principle of reduction and distortion of homologs in paleontology.
What do you mean by "gradually elaborated"? I doubt that you could find any
reputable evolutionary biologist who believes that individual vertebrae
originated independently, rather than by duplication.
>>it's accepted that small changes in
>>genotype can result in large changes in phenotype. I don't think that
random
>>blotches gradually became more and more linear and parallel until they
>>formed a striped pattern. I suspect that the first, crude striped pattern
>>resulted from a single mutation in a control gene.
>
>I suppose anything is possible for a "control gene", but even a crude
striped
>pattern seems a pretty complex (and symmetrical) thing for random mutation
>to produce.
A basic striped pattern isn't complex. If you imagine material
being continuously extruded from a machine, all that's needed to give it a
striped pattern is for the addition of pigment to the material to be
switched on and off at regular intervals. Now, of course, an animal's skin
doesn't grow in this way, and I don't suggest it would be quite a simple as
this, but the analogy shows that a striped pattern is not complex in
principle.
It seems to me that much of your problem with the conventional view is that
you're thinking almost entirely in terms of gross morphology, and not
sufficiently in terms of the genotype. The genome almost certainly doesn't
contain a blueprint for a striped pattern, or even instructions for coding
each stripe individually. It's more likely that there's a small amount of
code that sets up something like my pigment switch.
>Really, the tie-in to the skeleton makes much more sense.
Your supposed tie-in is extremely poor. If the stripes were originally tied
to skeletal segements, how did they become untied in places and migrate to
locations where they don't correspond to skeletal segments?
And how could the skeletal structure be reflected in the skin? What is the
physiological connection between skeletal segments and skin locations? (And
bear in mind that we're talking about ancient skeletal segments, not
present-day ones. I assume your explanation of multiple stripes per leg bone
is based on the assumption that the present day leg bones have fused from
mutiple smaller segments. By the way, since you like to use ontogeny to
support your arguments, does embryology show any sign of
fusing of limb bones from smaller segments?)
>As you
>may recall from the article, this is not my idea, I cite the articles in
>which this
>idea is argued for.
>I only bring it up as it supports my 'skeletal archetype'
>theory IMO.
Unfortunately, I don't have access to the sources you cited. Could you
briefly summarize the relevant points, please?
Richard Wein (Tich)
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