Murid rodents are only one of the approximately 146 families of mammals, yet comprise nearly one-third of all mammalian species. A robust phylogeny would provide the framework for understanding their evolutionary success as well as their roles as model organisms in biomedical research and as hosts and vectors of human pathogens. Steppan et al. present analyses based on sequences from 53 genera of four nuclear genes (GHR, BRCA1, RAG1, and c-myc), which yield nearly identical phylogenies. Taken together, these resolve most relationships among the 16 subfamilies and identify four distinct explosive radiations. One occurred when the ancestor of most Sigmodontinae colonized South America; another as the Murinae (Old World mice and rats) expanded their range from Southeast Asia across Asia and Africa. The results also suggest that--through the attribution of fossil calibrations to the wrong nodes and the neglect of rate heterogeneity--nearly all past applications of a molecular clock calibrated using the mouse/rat divergence have overestimated dates (that is, placed them too far back in time) by 20 to 50%. -- SJS
Syst. Biol.53, 533 (2004).
