From: Cliff Lundberg <cliff@cab.com>
>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.
Well, I'm not dissatisfied with the current model, but I also don't think
that the current model is the final word. The question is whether your
proposed model (or any other proposed model) is an improvement on the
current model. I don't really see that your model offers any significant
advantages over the current model, and I see some major disadvantages.
[...]
>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.
That remains to be seen, since you dismiss evidence to the contrary by ad
hoc arguments (as already discussed).
But let's suppose you're correct that no new segments have been formed
during post-Cambrian evolution. The question still remains: why do you
prefer Siamese twinning to duplication of segments by the Dawkinsian model
for *pre-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;
[...]
But Siamese twinning isn't heritable (is it?). Genetic duplication (by the
Dawkinsian model) is a better explanation precisely because it's heritable.
Perhaps it's much less likely to occur in the first place, but, once it does
occur, the change will be retained.
>>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.
Evolutionary biologists today no longer propose the kind of gradualism that
you keep attacking (if they ever did). It's accepted that small changes in
genotype can result in large changes in phenotype. I don't think that random
blotches gradually became more and more linear and parallel until they
formed a striped pattern. I suspect that the first, crude striped pattern
resulted from a single mutation in a control gene.
[...]
Richard Wein (Tich)
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