The Society for Integrative and Comparative Biology
Phylogenetic Context and Basal Metazoan Model Systems1
1 ITZ, Ecology & Evolution, TiHo Hannover, Bünteweg 17d, D-30559 Hannover, Germany
2 Department of Ecology and Evolutionary Biology, 1200 Sunnyside Ave, University of Kansas, Lawrence, Kansas 66045
3 Biology and Department, 1250 N. Dartmouth Ave., Harvey Mudd College, Claremont, California 91711
In comparative studies using model organisms, extant taxa are often referred to as basal. The term suggests that such taxa are descendants of lineages that diverged early in the history of some larger taxon. By this usage, the basal metazoans comprise just four phyla (Placozoa, Porifera, Cnidaria, and Ctenophora) and the large clade Bilateria. We advise against this practice because basal refers to a region at the base or root of a phylogenetic tree. Thus, referring to an extant taxon or species as basal, or as more basal than another, can be misleading. While much progress has been made toward understanding some of the phylogenetic relationships within these groups, the relationships among them are still largely not known with certainty. Thus, sound inferences from comparative studies of model organisms demand continued illumination of phylogeny. Hypotheses about the mechanisms underlying metazoan evolution can be drawn from the study of model organisms in Cnidaria, Ctenophora, Placozoa, and Porifera, but it is clear that these model organisms are likely to be derived in many respects. Therefore, testing these hypotheses requires the study of yet additional model organisms. The most effective tests are those that investigate model organisms with phylogenetic positions among two sister groups comprising a larger clade of interest.
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