Trends in Genetics
ReviewAnimal evolution: the end of the intermediate taxa?
Section snippets
A short history of metazoan molecular phylogeny
Metazoan molecular phylogeny is a relatively recent exercise whose beginnings can be dated rather precisely with the paper by Field et al.5 in 1988. There, the first broad phylogeny of animals, based on partial 18S rRNA sequences of 20 taxa belonging to 10 phyla, was proposed. This was an era of naive optimism and great expectations.
The first molecular trees displayed expected and surprising results. Diploblastic organisms were deeply split from triploblastic ones, to the point that the
Developmental and paleontological implications
The new phylogeny has several profound implications for understanding animal evolution. First, it suggests that the hope of finding extant ‘intermediates’ in the bilaterian lineage could well be doomed. We should consider the possibility that the tree might lead from diploblastic organisms to the coelomate bilaterians without any of the long-cherished intermediate phyla. This makes it more difficult to picture the origin of the bilaterian body plan but, at the same time, accounts much better
Acknowledgements
Parts of this paper were written while we were participating in the Embryology course at the Marine Biological Laboratory in Woods Hole. We thank the course and its organizers, S. Fraser and M. Bronner-Fraser, for the hospitality and facilities made available. We also thank H. Philippe and C. Nielsen for numerous discussions over phylogeny, J. Venutti and E. Davidson for many helpful comments. Finally, we thank D. Tautz for encouraging us to write this review. The work in the Orsay lab is
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