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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