Systematization of the Annelida: different approaches

Different approaches and different data sets are used to address questions about the phylogenetic relationships of the polychaete family taxa, the position of the Clitellata, the monophyletic nature of the Polychaeta and the Annelida, as well as the position of the Pogonophora and Echiura. In part, the authors of this contribution come to different conclusions regarding these questions: (1) The analysis of G. W. Rouse minimizes assumptions about any process concerning evolution in annelids and makes a detailed series of homology assessments, based on morphology, across the entire range of taxa concerned. A monophyletic taxon Polychaeta is found but there is no evidence of a sister group for the Clitellata among the polychaetes. The Pogonophora are positioned as members of the Polychaeta. A new classification of the polychaete families is provided. Problems with coding morphology for cladistic analysis are outlined and discussed. (2) D. McHugh uses the sequence of a nuclear gene, elongation factor-1α, for phylogenetic analysis by the parsimony and neighbour joining methods. The main message from phylogenetic analyses of molecular data is that there is no evidence for a monophyletic Polychaeta; instead, placement of the clitellates, pogonophorans and echiurans within the polychaete clade is supported, rendering the Polychaeta paraphyletic. (3) The phylogenetic system of W. Westheide and G. Purschke is based on Hennigian reasoning, and their character weighting is based on diverse, mainly functional considerations. Their annelid tree shows the Clitellata as a highly evolved monophylum and the polychaetes as paraphyletic. Special emphasis is placed on the demonstration that the Clitellata are of terrestrial origin. Advantages and disadvantages of the different approaches and open questions are discussed.