Cladistic

Cladistics is a method of systematic biology that classifies organisms based on their evolutionary relationships. The central idea is that groups should reflect common ancestry, forming clades that include an ancestor and all descendants. Cladistic analysis seeks to identify shared derived characters, or synapomorphies, that define these groups and distinguish one clade from another.

Key concepts include the distinction between monophyletic groups (clades) and other groupings. Synapomorphies are characters that arose in an ancestor and are inherited by its descendants, while plesiomorphies are ancestral traits shared by multiple groups. Homology refers to similarity due to shared ancestry, whereas homoplasy (convergence or reversal) can complicate interpretations. Outgroups are used to root trees and establish the polarity of character states.

In practice, cladistic work involves assembling a character matrix that codes the presence, absence, or state of traits for a set of taxa. Researchers select an outgroup, code characters, and use computational methods to infer cladograms—branching diagrams that depict relationships. Common analytical approaches include maximum parsimony, which seeks the simplest explanation with the fewest changes, as well as maximum likelihood and Bayesian methods that incorporate models of character evolution. Support for inferred relationships is often assessed with bootstrap or posterior probability values.

Data used in cladistics come from morphology, molecular sequences, or both. The resulting trees inform taxonomy, evolutionary biology, and comparative studies, and they continue to be refined as new data and methods become available. Cladistics, developed by Georgius G. G. Hennig in the mid-20th century, remains central to modern systematics.