Richard Wein wrote:
>I've read your web pages (fairly quickly). I found them well written, but I
>didn't find your theory at all convincing.
Thanks for taking the time. I'll respond to your objections. But it's
going to be hard for you to get into a theory like this unless you are
motivated by dissatisfaction with the current Dawkinsian model.
>The single biggest problem is that you describe evolution through merging of
>organisms entirely at the level of the phenotype. But, at some point, the
>structure of the composite organism must have have become encoded in the
>genotype. Without some hypothesis for how this might have happened, there's
>a gaping whole at the centre of your theory.
First, as I say in the article, it's a theory that could have and should have
been set forth in Darwin's time, before anything was know about genotypes.
It's primarily a morphological theory, and morphology was big in the 19th
Century. But the notion of simple-to-complex progressiveness was too
deeply ingrained among both creationist and evolutionist writers for this
somewhat contrary model to be entertained by either side at that time.
I agree that in the case of 'composite' organisms, as we discussed recently,
the would-be theorist should be obligated to attempt some explanation of how
this could happen genetically. This I did in a message, suggesting that zygotes
formed from gametes of different species would have to carry out endomitosis
to form a normal diploid cell to begin development; I also suggested that
things
were a little wilder in the past, when metazoan defenses against such bizarre
matings might not have existed, and when metazoans were not so well-evolved
that they would instantly wipe out such strange experiments. But in any case,
the article avoids mention of speculations about metazoans forming through
symbiosis. The only mechanism invoked is the comprehensible and familiar
one of Siamese-twinning (parabiosis is a possible synonym). The theory in the
article is about segmentation, not organismic complexity; it begins with an
unsegmented metazoan which has already attained modern metazoan
complexity.
>In the absence of such a hypothesis, why should I believe that this process
>is any more plausible than the conventional view that additional segments
>were created by mutations? You dismissed this view in the following terms:
>"Those who strive to explain elaborative evolution sometimes claim that
>number of segments in a train such as the vertebral column can be increased
>simply by a mutation that inserts complete segments into the series. Richard
>Dawkins suggests that such additions may be accomplished "easily", through
>"a simple process of duplication" (Dawkins 1986). I argue that such a
>mutation would be like parabiosis, but with the additional attached body
>accidentally reduced to the form of one perfect segment and accidentally
>positioned perfectly within a series of segments; this seems virtually
>impossible."
>
>You seem to be attacking a straw man here. Dawkins is not suggesting that
>the DNA code for the whole body is duplicated and then reduced. He's
>suggesting that the code for a single segment is duplicated. Indeed, even
>this may not be necessary. It's probable (but I'm not sure) that the code is
>not repeated for identical or near-identical segments, but that that such
>segments are coded once, with some gene controlling the number of times that
>the code is expressed. If that's the case, then only a mutation in the
>control gene is required.
There are indeed control genes, but I believe what they do is *limit* the
expression of a complex archetype. The crucial thing that the whole model
is based upon is the interesting *fact*, utterly ignored by Dawkins, that the
number of segments in metazoan lineages is diminishing. When you
accept this simple fact, you must postulate a beginning when segmented
organisms rapidly appeared, as the evidence shows most plainly.
I don't disagree that homologous segments or Siamese twins are formed
originally from the same genetic information, whether from the very same
sequence or from copies of that sequence. I do disagree about whether a
control gene can create brand-new segments; the evidence implies that
brand-new segments have not been formed during post-Cambrian evolution.
>Such extension by duplication would have been all the easier in the simpler
>organisms of the pre-Cambrian period, where you claim that all the new
>skeletal segments evolved.
Siamese-twinning is easy; it's the failure of multiple embryos to fully
differentiate; interestingly, it's feasible that this failure be caused by a
proverbial normally deleterious random mutation, yet the result is an
increase in the morphological complexity of the phenotype. What is
not easy is that a random mutation should add another segment within
an organism, to a finger, to a limb, to a spine or whatever. Again, along
with the theoretical difficulty is the fossil evidence, which only shows
reduction in number of segments, not elaboration.
>I must also say that I found your argument regarding the zebra's stripes
>utterly unconvincing. Many different camouflage patterns can be found on
>animals--stripes, spots and patches of many shapes and sizes. The fact that
>a small number of animals have striped patterns bearing a crude (extremely
>crude) resemblance to their skeletal structure strikes me as nothing more
>than a coincidence.
You really believe that the crisp striping of the zebra evolved through natural
selection acting upon random blotches of coloration? Well, I guess if you
think the complex symmetrical skeleton gradually was elaborated through
natural selection of slight random deviations in form, then you could think
that about stripes as well.
>I also notice you have a tendency to explain away any contrary evidence in
>the fossil record by ad hoc propositions of unknown types of organism. For
>example:
>"I suggest that many-segmented ancestors of snakes did exist in the
>Paleozoic Era but that we happen to lack fossil evidence of them, and that
>the pattern of reduction in number of segments does generally hold in
>vertebrate lineages after the formation of the protovertebrate. Snakes are
>unlikely to be fossilized, as they are adapted to move in mud, which can
>mire and entomb tetrapods or fishes."
The snake is hardly an unknown type of organism. What I say is definitely
ad hoc though. If one doesn't believe that the known organisms can be
lined up in order of increasing complexity to illustrate evolution, then one
has to postulate some unknown progenitors.
>I don't rule out parts of your theory as theoretical possibilities. But
>given its highly speculative and vague nature, I see no reason to overthrow
>the much more well-considered (and plausible IMO) conventional view. (And
>that's before we even start on the evolution of organs, for which you don't
>have even an outline of a scenario.)
The skeletal theory in the article is based on the fact of the reduction and
specialization of segments in vertebrate and arthropod lineages. In the case
of organs, there is no such supportive evidence, there's only the theoretical
argument irreducible complexity offers against gradual elaboration. For that
reason I'm careful not to saddle this segmentation theory with the more
speculative symbiosis theories.
Again, if you have no major problem with conventional theory, you will
not be motivated to get your head around anything radically different.
This isn't a simple syllogism that can demand acceptance. Deep habits
of thought are at stake.
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
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