>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.
So I'm wondering if you can point to experiments illuminating
what you mean by the "mechanisms by which new body plans develop."
Paul Nelson