From: Cliff Lundberg <cliff@cab.com>
>>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.
This comment, if you really mean it, shows a profound ignorance of biology.
I suggest you read a basic text on genetics and inheritance. Morphology per
se is certainly *not* heritable. Our morpholgy is determined by both genes
and environment ("nature and nurture"). Of these, only genes are inherited
(though our parents also play a role in our environment).
I spent a few hours today browsing the web for info on Siamese ("conjoint")
twins. Duplication by Siamese twinning is not heritable. The twins do not
receive two copies of the genome. Their genome is normal. Siamese twinning
is caused by environmental factors which result in a partial splitting of
the embryo. For example:
"If the notochord reaches an area of hypoxia, an area with low oxygen
content, the notochord will split to go to higher area of oxygen
concentration. This will cause you to have two notochords. Thus you will
induce two neural tubes. At term, the embryo will have two heads, two
brains, two spinal cords, one body. This is called a conjoint twin."
(http://www.musc.edu/related/comsc/embryo4.htm)
[In any case, if Siamese twinning was heritable, that would torpedo your
argument against Dawkinsian duplication. A heritable partial Siamese
twinning, where only some organs/limbs are duplicated, would be a type of
Dawkinsian duplication in a modern organism!]
And I come back to the point that I made earlier. Partial Siamese twinning
shows that a simple chance disruption during development can result in
functional duplicate limbs and organs, even in an organism as complex as a
human. If this can happen when the disruption is by chance, why can't it
also happen when the disruption is due to a simple mutation?
BTW here's the best web page I've been able to find on Siamese twinning:
http://www.thefetus.net/sections/articles/Multiple_gestations/Conjoined_twin
s_structural_anomalies_Jaffe.html
>>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?
You don't describe the mechanisms. You describe only the morphology. The
mechanisms of evolution are primarily random mutation and natural
selection. You hardly address these vital issues.
>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.
You're missing my point. The point of this part of my argument (the context
has now been lost) was to establish that some segments of your early forms
must have been generated from one code segment repeatedly processed. Do you
now agree with that?
>>>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.
On the contrary, it's vital to distinguish hypotheses from facts, even if
that means a little extra verbosity. What's really tiresome for me is having
to keep checking whether your statements are hypotheses or 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.
>
>I don't see how Siamese-twinning duplication creates less complex
>segments. What would be more complex than multiplying the whole
>body?
You misunderstood me (refer to the original context). I'm saying that both
your model and mine require the duplication of segments by Dawkinsian
processes. But the segments duplicated in your model are less complex than
the segments duplicated in mine.
>>>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.
This requires an astonishing degree of coincidence.
>>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.
OK. I thought you were talking about organs. I have no problem with nearby
muscles being co-opted into the formation of an eye. If you're not involving
other organs, then I think we have a pretty similar idea of the evolution of
the eye. By the way, I assume you are allowing duplication of
light-sensitive cells and their nerve connections. (I just want to point out
again that you do require Dawkinsian duplication of simple structures.)
>>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.
It's not enough to rely on random success. For changes to be retained and
improved on, you need a selective advantage. It's not plausible that a
relatively complex structure like your proto-vertebrate evolved by random
mutation without natural selection.
>>>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?
Of course, I don't know any more than you know how and when your fusion
occurred.
>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're talking about fusion of individual organs (not the whole organism)
into a functioning single organ. This involves the same kind of difficulties
as duplicating an organ.
>>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.
Even though they're pointing in different directions? Get real!
>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?
Most of the photos I've seen had stripes in various orientations, such as
||||||||||||||||||||| and //////////////\\\\\\\\\\\\\\\\. But I think there
were some with odd shapes too. It probably pays to vary the pattern, to keep
the enemy confused.
Coming back to the subject of extra toes and fingers (polydactyly), I did
some research. It's interesting that polydactyly is associated with over 30
different congenital disorders, having a wide variety of effects
(http://www.drgreene.com/970519.asp). One of these, Trisomy 13, results
from the duplication of the whole of chromosome 13. This doesn't
sound like reactivation of an atavistic gene.
Furthermore, an additional thumb is possible
(http://www.medmedia.com/o15/113.htm). Are you going to argue that an
ancestor had two thumbs per hand? What's more, it isn't necessarily the
whole thumb that's repeated, sometimes it's just part, or even just the tip.
Yet all of these variations seem to be viable, i.e. they at least have a
functioning circulatory system. It's odd how all these different atavistic
genes function so well in conjunction with the other genes for a modern
hand, which must be very different from the atavistic precursor hand.
Richard Wein (Tich)
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