Few supraspecific groups have been defined in North American ranids and the informal groupings which are recognized are often poorly characterized. Two biochemical methods, starch gel electrophoresis and microcomplement fixation, have been used in an examination of the evolutionary relationships among western North American frogs of the genus Rana. Both the electrophoretic and albumin comparisons indicate that the Rana boylii species group presently includes two very different evolutionary lineages. Rana aurora, R. boylii, R. cascadae, R. muscosa, and R. pretiosa are all members of one lineage allied to R. temporaria of Europe. A Mexican species traditionally included in this group, R. tarahumarae, is most closely related to other members of the genus that occur in Mexico and is part of a larger lineage that also includes R. pipiens. Frogs found in eastern North America diverged from western European frogs in mid-Eocene; estimates of divergence time are consistent with the hypothesis that separation of these lineages coincided with the end of a land connection between Europe and North America. The catesbeiana, pipiens, and tarahumarae groups diverged from each other in the Oligocene. Western North American Rana diverged from a Eurasian ancestor in the Oligocene and radiated in this area to form the five members of the boylii group.
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