Cladistics

Cladistics is a method of biological classification that groups organisms by their common ancestry, yielding clades that include a common ancestor and all its descendants. Clades are defined by shared derived characteristics, or synapomorphies, that diagnose a lineage. In a cladistic analysis, characters are coded for multiple taxa and polarity is established (ancestral versus derived) using an outgroup. Homology is the basis for recognizing traits shared by descent, while homoplasy—convergence or reversal—can complicate inferences and lead to misleading similarities if not properly accounted for.

The approach was formalized by Willi Hennig in the 1950s as an attempt to reflect evolutionary history more faithfully than traditional, similarity-based taxonomy. Cladograms are branching diagrams that depict hypothesized relationships; they are typically rooted and show the order of divergences. Methodologically, cladistics has evolved from parsimony, which seeks trees with the fewest character changes, to methods such as maximum likelihood and Bayesian inference that incorporate explicit models of character evolution, especially with molecular data.

A central implication is the emphasis on monophyletic groups—units that include all descendants of a common ancestor. This framework often reshapes taxonomy, rejecting paraphyletic or polyphyletic groups. For example, many cladistic analyses place birds within theropod dinosaurs, making Aves a subset of Dinosauria. Limitations include sensitivity to chosen characters and data availability, the persistent problem of convergent evolution, and incomplete fossil records, all of which can yield uncertain or contested relationships despite the cladistic approach.