Roger Thomas presented a GSA abstract in which he systematically considered
various shapes to see if any Cambian organism had that shape and if any
later organism had that shape. I'll check if it has been published.
>>Since the Cambrian, no mass extinction has
>>been massive enough to provide as many possible body plans not already
>>being used.
>
>Huh?
That would probably be better in more than one sentence. No mass
extinction has entirely wiped out all representatives of a phylum-level
body plan. Therefore, there has been no opportunity to reinvent such a
plan without pre-existing competition. Only when invading a previously
empty niche has much novelty arisen.
>
> If something with a given body plan, lifestyle, etc. already
>>exists, it's going to be difficult for something else to evolve into the
>>same niche. The new kind is unlikely to be as good as the existing kind at
>>what the existing one does, so it will get outcompeted and not evolve in
>>that direction.
>
>How then could 600 species of Drosophila possibly evolve in the Hawaiian
>Islands?
This is variation within flies. If a Drosophilia "tried" to evolve a
beetle-like form in an environment that already had beetles, it probably
would be outcompeted in the half-fly half-beetle stage. The beetles are
already good at beetling, but the fly is not. If Drosophilia encountered
an open niche, it might evolve into something rather different, such as the
wingless bat symbiotes in New Zealand. (They live on the bats and feed on
guano-associated fungi, if I recall correctly-they aren't parasites).
David Campbell