From: Cliff Lundberg <cliff@cab.com>
>>>--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.
I agree that a description of post-Cambrian evolution *doesn't* have to say
anything about the mechanism of Cambrian/pre-Cambrian evolution. I just felt
it wasn't clear that you were keeping the two separate.
Your hypothesis can be divided into 2 parts:
A) During the Cambrian explosion, new segments were added in the manner you
describe.
B) Evolution since the Cambrian has been by reduction and specialization
only, with no new segments.
You say that your theory explains the nature of post-Cambrian evolution,
i.e. the absence of new segments in the post-Cambrian. But this absence does
not follow from A, and to say that it follows from B is circular reasoning.
The absence of new segments is not a known fact--it's part of your
hypothesis.
[...]
>>>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.
You *claim* that it's erroneous. But we don't know that.
>>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.
A trend is just that. It may have exceptions.
>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.
[...]
>>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.
Are you saying that genetic birth defects are the same thing as
Siamese-twinning?
>>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.
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?
>>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.
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.
>>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.
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.
>>- 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.
*Beginning* to take on sensory roles? I thought you didn't accept the
existence of incipient organs! ;-)
>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.)
>>- 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.
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?
>>- 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.
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.
>>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.
There's no reason to think that the addition of segments in the shapes you
describe would give an increase in mobility at each stage. Remember that
evolution by natural selection cannot look ahead to benefits that might be
acquired at a future stage. Reverse engineering is fine. But each step in
the process must be plausible.
>>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.
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.
>>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.
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.
>>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.
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.
>They're
>probably distorted sets of vertical stripes,
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.
>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?
I'm afraid that these arguments of yours are starting to take on a distinct
air of desperation!
Richard Wein (Tich)
This archive was generated by hypermail 2b29 : Sat Jun 17 2000 - 10:25:15 EDT