Problems and paradigms: A mammalian molecular clock?

The molecular clock hypothesis remains controversial more than a quarter of a century after it was first proposed. A variety of approaches have been applied to testing the molecular clock in mammals. In many of these studies apparent refutation of the molecular clock has based on false assumptions about the pattern of mammalian evolution. With a few exceptions there now appears to be little evidence for variation in the rate of molecular evolution among mammalian lineages, although comparison of more genes and proteins among a greater variety of mammalian orders is needed before a definitive conclusion can be reached on the issue.