Cladistics

Cladistics is a method of biological classification that groups organisms strictly by common ancestry, rejecting similarities that arose through convergent evolution. Developed by the German entomologist Willi Hennig in the mid-twentieth century, cladistics revolutionized systematics by making phylogenetic hypotheses explicit and testable. A clade is a group consisting of a single ancestor and all its descendants; only monophyletic groups are considered valid. This rigidity makes cladistics powerful but also controversial — it can force the revision of familiar groupings (such as excluding birds from reptiles, or recognizing that whales are deeply nested within mammals). Cladistics remains the philosophical backbone of modern phylogenetics, even as the data and methods have shifted from morphology to molecular sequences. Its relationship to taxonomy remains contested: cladistics demands purity, while taxonomy must balance history with practical utility.