Model organisms are often assumed to be representative of some more inclusive taxon of which the species is a part. This assumption leads to mistaken generalizations about the evolutionary and comparative significance of the data gathered. This paper reviews com? parative and evolutionary studies of Xenopus laevis and its relatives. Phylogenetic analysis of data from DNA sequences and morphology indicate that Xenopus is monophyletic and that Silurana is its sister group. The most basal lineages of Pipidae diverged prior to the breakup of Gondwana. The bizarre morphology of Xenopus is in part due to changes in the mode of meta? morphosis. Speciation in Xenopus is unique among Anura in being associated with various levels of polyploidy owing to allopolyploidy. Several kinds of molecular studies indicate substantial divergence between Xenopus and Silurana. The contribution of data from model studies of Xenopus would be greatly enhanced if comparable data were available from a more basally placed lineage such as Bombina. [Xenopus; Silurana; Pipidae; model organisms; morphology; historical biogeog- raphy; fossils; molecular systematics; ribosomal DNA; mitochondrial DNA; phylogeny; devel? opment; heterochrony; caenogenesis; functional anatomy; behavior; polyploidy; karyotypes; molecular biology.]

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