Short notes and reviews Carrying metazoan phylogenetics forward in the 21st century

Review of and commentary on: Animal Evolution. Interrelationships of the Living Phyla, by Claus Nielsen. Oxford University Press, 2001, x + 563 pp., ISBN 0-19-850682-1 (Pbk). My initial browsing of the new edition of Claus Nielsen’s Animal Evolution could scarcely have produced a bigger surprise. Opening the book from the back I first read the last line of the last chapter on molecular phylogeny: “The evolutionary scenario pictured in fig. 57.2 [a molecular cladogram based on 18S rDNA sequences] is fully compatible with the phylogeny proposed on morphological evidence in this book...” (p. 519). Imagine the surprise! Anyone even marginally familiar with recent developments in higher-level animal phylogenetics will be aware of some major conflicting phylogenetic signals lurking below the surface of morphological and molecular data sets. Certainly, many may agree with Balter’s (1997) observation that “morphologists learn to live with molecular upstarts,” but in view of Nielsen’s less than enthusiastic embrace of molecular phylogenetic evidence in previous publications (Nielsen, 1997a, b) he appears particularly unlikely to serve as a candidate molecular convert. However, a glance at Nielsen’s molecular ‘tree’ offers immediate clarification. It resembles a three-layered terrace overgrown with a phylogenetic grass reminiscent of the massively polyphyletic scheme advocated a decade ago by Willmer(1990), which enjoyed some notoriety as the then most recent pictorial incarnation of phylogenetic ignorance. Nielsen’s rendition of IBS rDNA sequence data merely yields three phylogenetic plateaus representing the ascending organizational levels of Metazoa, Bilateria, and Deuterostomia forming the substrates of three successive Precambrian radiations. Unsurprisingly, the complete lack of resolution in the molecular phylogeny-guarantees compatibility with any morphological tree. Needless to say, others have reached rather different conclusions about the degree of phylogenetic resolution that 18S data can offer. In fact, where most phylogeneticists (including myself) are prepared to defend more resolution for inter-phylum relationships, Nielsen’s molecular cladogram offers resolution where 18S data do not provide it, namely for the monophyly of several of the individual phyla. Nielsen’s fig. 57.1 nicely illustrates this shortcoming of IBS data to support the monophyly of even the morphologically bestdefined phyla such as Mollusca. Nielsen very briefly touches upon some other issues with molecular phylogenetics that have attracted widespread attention in the recent literature, such as the difficulty of estimating divergence times, and the conflict or congruence between phylogenetic estimates based on different molecules. A short section on evolutionary developmental biology is also provided, but readers who are seriously interested in molecular phylogenetics are best advised to consult the extensive primary literature as Nielsen himself candidly recommends.