Hi, Hoot Mon
Hoot Mon writes:
But trochophore larvae are not monophyletic (again, per Williamson & Vickers' diagram):
I have always read that all the phyla in that diagram (Sipuncula, Mollusca, Annelida and Rotifera) are grouped in the clade Lophotrochozoa, whose common ancestor was proposed to have had a trochophore larva.
I realize that there is some disagreement as to the placement of the phylum Rotifera, which compounds the matter significantly. But, I don't know enough about this subject to make an informed conclusion.
I remain skeptical of this larval-hybridization hypothesis, simply because the two alternatives (monophyly and convergence) are apparently at least as robust as it is. It could be tested, I think: you could sequence genes that are active during larval development, and separately sequence adult genes, and plot separate cladograms based on adult and larval genes. If the larval cladogram resolves a trochophore clade while the adult cladogram does not, this could support Williamson's hypothesis. Granted, the process would be a pain in the butt, but the technology exists (I think).
If Williamson's hypothesis turned out to be correct, it would be a very interesting development in evolutionary biology. At this point, it seems to be a violation of parsimony to me, but I'll withhold final judgment until it has been tested.
-Bluejay
Darwin loves you.