Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants (and nothing else). For example, birds, dinosaurs, crocodiles, and all descendants (living or extinct) of their most recent common ancestor form a clade.

Cladistically, snakes are lizards, termites are cockroaches, fleas are scorpionflies, birds are dinosaurs, and humans are apes.