Kladistik

Kladistik (cladistics) is a method in biological systematics that aims to reconstruct evolutionary relationships by grouping organisms into clades—monophyletic groups defined by shared derived characters, or synapomorphies, that indicate common ancestry. The resulting hypothesis of relationships is represented as a branching diagram called a cladogram or, in some contexts, a phylogenetic tree.

Core ideas include distinguishing homologous traits from analogous similarities, and the emphasis on monophyly as the criterion for natural groups. Synapomorphies define clades, while plesiomorphies and autapomorphies are treated differently. Paraphyletic and polyphyletic groups are considered artificial under strict cladistic criteria.

History and development: The method was formulated by Willi Hennig in the mid-20th century as a critique of phenetics, arguing that taxonomic similarity should reflect shared evolutionary history. Hennig's work popularized a formal, character-based approach to systematics, which has since spread worldwide.

Data and methods: Analysts code characters (morphological, molecular, or behavioral) into discrete states and use computational methods to infer the most plausible tree. Common approaches include maximum parsimony, maximum likelihood, and Bayesian inference. Outgroups are used to root the tree, and support for clades is assessed with measures such as bootstrap values or posterior probabilities.

Applications and limitations: Cladistics informs taxonomy, comparative biology, and evolutionary studies by focusing on clade-based classifications. Practical debates concern character selection, model assumptions, and conflicting signals from convergent evolution or incomplete lineage sorting. Despite limitations, cladistics remains a foundational framework in modern systematics.