Did you have to ask a question on molecular genetics where I am but an
interested spectator? Only kidding! I will do my best.
pnelson2@ix.netcom.com wrote:
> Jonathan Clarke wrote, in part:
>
> >So in about 25 million years, less than half the length
> >of the Tertiary, we see the appearance of all the skeletonised
> >phyla. There is no other period in the fossil record like it.
> >
> >The problem is not intractable. Molecular genetics can give
> >us phylogenies and mechanisms by which new body plans develop.
> >
> >[...]
> >
> >We know a lot more than we did 20 years
> >ago, especially about how changes body plans happen at the level
> >of developmental biology and genetics [...]
>
> I wonder if you could say more, Jonathan, about how you see
> molecular genetics having illuminated how new body plans develop,
> or how they change viably.
>
> I ask because I see the situation quite differently. Saturation
> mutagenesis in both Drosophila and zebrafish, and indeed the
> perturbation of development in all known model metazoans, gives
> no evidence that these body plans ever want to change (so to speak).
> Indeed, the absence of viable mutants which might provide the
> basis of higher-level evolution (at the origin-of-novel-body-plans
> level) has led Gould and others (e.g., Douglas Erwin) to propose
> profound temporal asymmetries in evolutionary processes. In brief,
> ontogenies have now "hardened" irreversibly; so that a fruit fly
> or zebrafish population under mutagenesis nowadays will die, sicken,
> or otherwise tumble off its adaptive peak -- but things were very
> different in the Cambrian radiation. Ontogenies were labile and
> permitted rapid morphological change.
>
> Whatever that means.
I think you are correct. We don't see phyla-level body plans evolving
today, we haven't seen this since the beginning of the Ordovician.
>
>
> So I'm wondering if you can point to experiments illuminating
> what you mean by the "mechanisms by which new body plans develop."
I was referring to the experiments on the effects of Hox gene mutations,
which from memory (it is early morning and there is no way I can get to
a source for this) appear to be able to effect major changes in body
plan. Some appear to be able to effect changes at the phyletic level
(segmentation, endoskeletons vs exoskeletons etc.). As you point out,
the results of the experiments are non-viable, whether this was always
the case, remains to be seen. I should have said something like "the
way new body plans developed".
It is a matter of degree though. I don't see any reason why leglessness
should not develop now, and many lizards seem to be doing so. This
requires Hox gene mutation also.
Hope this clarifies my tortuous explanations of my muddy mental
processes.
>
>
> Paul Nelson
God Bless
Jonathan