Richard Wein wrote:
>>--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 sudden doubling-or-greater multiplication of a body is a morphological
macromutation. Prototypical arthropods and vertebrates form through this
sudden agglutination, followed by increasingly refined reduction and
distortion of the complex. Conventional theory envisions new segments budding
out to elaborate the skeleton, something I don't see in the fossil evidence and
have theoretical problems with. I guess this could happen almost as quickly as
my alternative, if it could happen at all, but I don't think it can.
>>--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.
Novel in the sense of being new segments. Morphological novelty continues
to develop gradually through reduction and distortion of segments. I don't
grasp why a description of post-Cambrian evolution has to say anything
about the mechanism of Cambrian/pre-Cambrian evolution; I'm pretty
specific about my general segmented-animal-forming mechanism for
that time.
>>--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!
That is your judgment. I wonder what the conventional explanation is
for lines of flexure in the skin of the unborn, for example. I guess if you
think the stripes of a zebra are evolved through RM&NS acting upon
blotches of color, you can think that natural selection gradually created
and positioned these lines, giving an advantage over competitors who
came into the world lacking pre-creased skin; but I'd have to think that
was an awfully low priority item for a species to be selecting for.
>>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.
'Ad hoc' doesn't seem apt in reference to such a presumptuously general
theory. As I said, parsimony is a factor you look to when you are grasping.
When you can see the plain reality before you, you simply describe it as it is,
with no regard for how parsimonious the accurate description may be.
>>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 meant to offer this as an example of a traditional idea which is erroneous.
>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.
The principle of reduction and distortion within sets of serial homologs
was established long ago in paleontology, established quietly and
reluctantly because it is counter-intuitive to progressive open-ended
evolution. Whatever evidence the generalization was based on, that's the
evidence. Apparent individual counterexamples notwithstanding, this
is the great trend. Polydactyly is an atavism, as are extra legs in insects.
>>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.)
His only comment that I know of is that the reduction and specialization
among homologous parts was a tiresome generalization, the result of a
lot of labor, but meaningless, because it fits no explanatory model. A
citation is in the article. Obviously it fits my model, as my model is based
upon it.
>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?
Not far-fetched at all, that's a highly popular view, but still wrong. The
proposal
is that the primitive progenitors were masses of segments, more elaborate
in that sense than any of their descendant organisms, and that their subsequent
evolution is a matter of reduction and distortion among those segments. This
is all their mutations can do, because there is no mechanism for inserting
brand
new segments in the pattern. Nor does explanation of the evidence require one.
>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.
Unfortunately, there are genes for birth defects.
>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.)
A morphological description is all I have in mind, that is, 'Siamese-twinning.'
As I keep saying, this is a morphological theory that should have been promoted
before genetics was even discovered, but evidently it was too counter-intuitive
for the progressive evolutionary mentality, and no doubt the paleontological
evidence for reduction and distortion was inferior then.
>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.
No comment, except that is intriguing that the number of segments in the
progenitors is in the same ballpark as the number of redundant sequences
in the genome (high hundreds to low thousands)
>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.
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.
>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...
Well, a new theory should lead to new problems. I think the Dawkinsian model
allowing gradual elaboration in number of segments is not valid, not supported
by evidence, and fraught with theoretical practical difficulty. Of course, in a
general sense, gradual evolution produces all kinds of new complexity,
but in the sense of number of segments, it doesn't produce any.
>- 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?
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.
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.
>- 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.
The process of fusion and distortion is Darwinian. I'm starting to think of
'Darwinian' as being something more benign than 'Dawkinsian', the latter
being less conservative about the limitations of RM&NS. But this usage
is no doubt too idiosyncratic.
>- 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.)
New parts come only through the crude mechanism of something-like-
Siamese-twinning, wherein entire bodies clump into one. All the subsequent
evolution streamlining the mass into an efficient organism is Darwinian.
The 'limbs' are created instantly through S-t; fusion and coordination of
systems with those of the axial skeleton is gradual. A somewhat
non-Dawkinsian aspect of this is the limitation against new segments
being added.
>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.
Selection should favor the larger and the more mobile. I confess to doing
reverse engineering. That is something you do when you try to reconstruct
the course of evolution.
>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.
Right, they recognize that they are homologs, built with the same genetic
information. But are they ever individually added, except through atavism,
which is not really adding something new?
>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.
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.
>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?
Morphology is not tied down, things migrate and distort during development
and evolution, in selected ways.
>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?)
Maybe not, but I don't see that as fatal. Certainly the skull forms from many
small pieces. It is pretty well accepted that our limbs have evolved from a
many-segmented lungfish limb; the history of individual bones can be
traced through evolution, whether or not they appear in modern embryos.
The limbs just aren't as revealing in the embryo as is the axial skeleton.
>Unfortunately, I don't have access to the sources you cited. Could you
>briefly summarize the relevant points, please?
Each somite, each fundamental embryonic segment, has a pigmented bit
and a non-pigmented bit. Cell growth of the skin traces back clonally to the
embryonic segments, one stripe per segment; each segment has one
region of melanocyte activity and one of inactivity which are apparent when
skin is formed. This mapping can be greatly distorted and ineffective,
resulting in no pattern marking. Or it can be strikingly reflected, as in the
zebra.
>One more point about stripes. If you think that stripes can only be
>explained in relation to skeletal segmentation, then how do you explain
>species with longitudinal stripes?
>I've just been glancing through Encarta, and it's not hard to find such
>species. A particularly good example is the common garter snake, which has 3
>clear stripes running the full length of its body. A lot of caterpillars and
>fish have longitudinal stripes. But also a few mammals, such as chipmunks
>and squirrels.
Longitudinal stripes are a lot simpler than zebra striping patterns. They're
probably distorted sets of vertical stripes, but they may be simple enough
to have evolved in other ways.
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
This archive was generated by hypermail 2b29 : Sat Jun 17 2000 - 02:29:06 EDT