The relative rate test is a genetic comparative test between two ingroups (somewhat closely related species) and an outgroup or “reference species” to compare mutation and evolutionary rates between the species.[1] Each ingroup species is compared independently to the outgroup to determine how closely related the two species are without knowing the exact time of divergence from their closest common ancestor.[2] If more change has occurred on one lineage relative to another lineage since their shared common ancestor, then the outgroup species will be more different from the faster-evolving lineage's species than it is from the slower-evolving lineage's species. This is because the faster-evolving lineage will, by definition, have accumulated more differences since the common ancestor than the slower-evolving lineage. This method can be applied to averaged data (i.e., groups of molecules), or individual molecules. It is possible for individual molecules to show evidence of approximately constant rates of change in different lineages even while the rates differ between different molecules. The relative rate test is a direct internal test of the molecular clock, for a given molecule and a given set of species, and shows that the molecular clock does not need to be (and should never be) assumed: It can be directly assessed from the data itself. Note that the logic can also be applied to any kind of data for which a distance measure can be defined (e.g., even morphological features).