Hi Cliff,
You don't seem to be grasping my arguments. Maybe I'm not expressing myself
clearly enough. So let me just reiterate my main point, and then I'll call
it a day. My main point is that you're concentrating almost entirely on
morphology while failing to consider the genetic and selection mechanisms
that are required.
Thanks for the interesting discussion.
Richard Wein.
-----Original Message-----
From: Cliff Lundberg <cliff@cab.com>
To: evolution@calvin.edu <evolution@calvin.edu>
Date: 21 June 2000 09:57
Subject: Re: macroevolution or macromutations?
>Richard Wein wrote:
>
>>So are you withdrawing your claim that the mechanism underlying your
theory
>>(heritable Siamese-twinning) is observed in humans?
>
>Here's what you read:
>
> Parabiosis is a familiar phenomenon, observable even in our own
species.
>It
> may be viewed as the failure of multiple embryos to differentiate
fully.
>Thus while
> parabiosis creates a more complex organism, it is in a sense a
reduction,
>in that
> it reflects a failure of normal embryonic differentiation, a
truncation
>of the normal
> course of development. Such degeneration may reasonably be attributed
to
>the
> proverbial "normally deleterious" random mutation.
>
>Humans are just another species of metazoan for my purposes here. If you
want
>to make the strong claim that parabiosis or Siamese-twinning is not
observable
>in humans, you might as well claim that it's not observable in any species.
You
>
>may think I'm implying that the phenomenon cannot be caused by
non-heritable
>physical circumstances during development, but it was not my intention to
deny
>that obvious possibility.
>
>>And since you agree that heritability of Siamese twinning is necessary,
>>*how* do you think it's inherited? I mentioned the possibility of
>>duplication of the whole genome, but you didn't seem to like that. What
>>mechanism do you propose?
>
>A mutation somehow screws up the process of embryonic differentiation, such
>that multiple embryos fail to differentiate fully. The 'twins' (for
simplicity)
>are
>viable and successful as a composite organism. They have the same genome,
>and carry the gene which caused their condition, so their offspring are
also
>joined twins.
>
>>>When I said "duplicate limbs and groups of organs, fine", I was only
>>>agreeing that a Siamese twin may consist of part of the complete
>>>organism, and that this part may include some limbs, some organs,
>>>some whatever. But this is too crude a mechanism to insert useful
>>>additional segments or organs into an organism, too crude to usefully
>>>elaborate morphology.
>>
>>Hang on. You just said that a Siamese twin may include some organs. Then
you
>>said that this mechanism is too crude to insert useful organs. What does
>>this mean? Are you saying that a Siamese twin can include a useless organ
>>but not a useful one? But we see Siamese twins with fully functional
>>duplicate organs.
>
>By 'insertion' of an organ, I meant a Siamese-twinning event in which one
of
>the siblings is (a) so perfectly reduced by chance that it consists of only
one
>
>organ or one skeletal segment, and (b) this one element is usefully fitted
into
>
>the anatomy of the other, whole, sibling. I don't see that as likely enough
to
>consider, mainly because of the unlikeliness of (a).
>
>>>The two-spine mutation you mentioned, for example, I don't see that
>>>succeeding.
>
>>Why not? If this deformity can result from a simple environmental
variation,
>>such as the presence of an area of low oxygen content, then why is it so
>>hard to believe that a similar deformity could result from a mutation?
>
>I was thinking of its success in the real world as an organism, not the
>success of a gene or a mechanical process in realizing this nonviable
>teratism. I have no problem believing that more things than I know of
>are capable of causing deformities. My Siamese-twinning takes place
>among simple metazoans to begin the Cambrian explosion, to form
>vertebrates and arthropods etc; I don't see this as an evolutionary
>mechanism for subsequent evolution.
>
>>>I don't see how a Siamese twin
>>>can be so fortuitously reduced and positioned so as to comprise one
>>>new functional item within a set of homologs in its sibling. I don't
>>>think that a hexadactyl individual could result from having 4 identical
>>>twin siblings, each reduced to one digit and positioned correctly. It's
not
>>>impossible, but it's astronomically unlikely.
>>
>>You've missed my point. I'm not saying that hexydactyly results from
Siamese
>>twinning. Please re-read my point above ("And I come back to the point
that
>>I made earlier...").
>>
>>>My model relies on regulatory genes to shape the organism from a
>>>progenitor which is informationally simple but complex in terms of
>>>number of parts. The information for the symmetrical progenitor
>>>with many parts remains, but regulatory genes limit and distort its
>>>expression during development. Perhaps there is room for agreement
>>>there, perhaps these are your 'control genes'.
>>
>>Yes, "regulatory genes" seems to be the more common term, though I've also
>>seen "control genes".
>
>Fine with me. But not if you claim that control genes can add brand-new
>non-atavistic segments to the skeleton of an arthropod or vertebrate.
>Through regulation the primordial form's expression is distorted and
>reduced, but it isn't elaborated in terms of number of segments.
>I try to explain what it is that is being regulated.
>
>--
>Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
>
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