Cladistics

Cladistics is a method of biological classification that groups organisms into clades, defined as a common ancestor together with all of its descendants, based on shared derived characteristics rather than overall similarity. German entomologist Willi Hennig set out the method's formal logic in a 1950 book written in German, later translated and expanded into the influential 1966 English edition Phylogenetic Systematics. The method represents evolutionary relationships as branching diagrams called cladograms, and its central principle is that only shared derived traits, not shared ancestral traits or independently evolved similarities, are valid evidence of common descent. Cladistics became the dominant framework in systematics and taxonomy from the late twentieth century onward, particularly once computational methods made it practical to analyze large character and genetic datasets, and it reshaped the classification of many groups by requiring that named taxa correspond to genuinely monophyletic groups.