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. 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. 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 slo