Phylogeny and Systematics

TLDR: The most recent common ancestor of two organisms generally is not used to determine the level of the taxonomic hierarchy at which they are separated and one reason for this apparent anomaly is that systematists who originally produced the hierarchical classifications in use today ranged along an axis from lumpers to splitters.

Abstract: There is near-universal acceptance that the way organisms are classified hierarchically into species, genera, tribes, families, orders, and so on should ideally describe a hierarchy of identity by descent from common ancestors. For example, a species compared with another in the same genus would be expected to have had a more recent common ancestor than that same species compared with another from a different genus. Perfectly understandably, however, there is a general reluctance to repeatedly revise previously accepted classifications unless there is a very good reason to do so. If a classification can be shown not to reflect a hierarchy of common ancestry then that would be accepted as a good reason for change: if a species in one genus had a more recent common ancestor with a species in another genus than it did with its supposed congeners, then biologists would revise their classification. Molecular systematics has revolutionized progress in identifying the most likely correct hierarchical classification. In contrast, the date of the most recent common ancestor of two organisms generally is not used to determine the level of the taxonomic hierarchy at which they are separated. The most recent common ancestor for two species in the same genus would have been earlier in some orders than the most recent common ancestor for two species separated at the family level in other orders. One reason for this apparent anomaly is that systematists who originally produced the hierarchical classifications in use today ranged along an axis from lumpers to splitters: the former were less keen to classify species with a particular degree of morphological similarity into different higher-level taxa than were the latter.