Definition
Cladistics is a method for reconstructing evolutionary relationships using shared derived characters — traits that arose in a common ancestor and are inherited by all its descendants. The unit of analysis is the clade: an ancestor and all of its descendants. Two species are placed together because they share a derived trait, not because they look similar overall.
Developed by Willi Hennig in the 1950s and 60s, cladistics replaced the older, more impressionistic methods of evolutionary systematics. It demands explicit character coding, repeatable algorithms, and an audit trail from data to tree.
Why it matters
How it works
The analyst builds a character matrix — rows for taxa, columns for traits, cells for character states (e.g. canine large, canine reduced, canine very small). One or more outgroup taxa establish which states are ancestral and which are derived. The matrix is then submitted to a tree-search algorithm that finds the tree explaining the data with the fewest evolutionary changes (parsimony) or the highest likelihood under an explicit model.
Results are usually reported as a most-parsimonious tree with bootstrap or posterior support values for each clade, telling readers how robust each grouping is to data resampling. Newer methods extend cladistics to molecular data, but the underlying logic — group by shared derived characters — is unchanged.