The biochemical phylogeny of guinea-pigs and gundis, and the paraphyly of the order rodentia.

1. Molecular data indicate that caviomorphs (guinea-pig-like rodents) and myomorphs (rat-like rodents) are not monophyletic. 2. Rather, the evolutionary lineage leading to the guinea-pig may have branched off prior to the divergence among myomorphs, lagomorphs, primates, chiropterans, artiodactyls, and carnivores. 3. Thus, the guinea-pig lineage probably represents an ancient eutherian lineage, and should be conferred an independent ordinal status. 4. The gundis (Ctenodactylidae) also seem to have branched off before the divergence among myomorphs, primates, and artiodactyls, but after the divergence of the guinea-pig. 5. Therefore, the order Rodentia as defined at the present time is in all probability a paraphyletic group devoid of taxonomic validity. 6. Previous claims pertaining to large differences in the rate of molecular evolution between guinea-pigs and myomorphs may have been exaggerated in many cases as a result of the erroneous phylogenetic position attributed to the guinea-pig. 7. The average rate of amino acid replacement in the guinea-pig is comparable to that in the rat and the mouse. 8. Protein-coding genes of myomorphs and caviomorphs evole, on average, about two times faster than their counterparts in gundis and humans.