Clade

A clade is a group of organisms that includes a common ancestor and all of its descendants, forming a single, natural branch on the tree of life. The term is rooted in cladistics, a method of classifying organisms by their evolutionary relationships, and derives from the Greek klados, meaning branch. In practice, clades are often depicted as nested branches on phylogenetic trees.

Clades are defined as monophyletic groups: they contain the most recent common ancestor of the members and all its descendants. They are identified through shared derived characters, or synapomorphies, but the defining criterion is ancestry rather than mere similarity. Clades can vary greatly in size and can be nested within larger clades, illustrating the hierarchical nature of evolutionary history.

Not all groupings are clades. Paraphyletic groups include a common ancestor and some, but not all, of its descendants (for example, traditional reptile groups that exclude birds). Polyphyletic groups assemble organisms from different ancestors, not reflecting a single common origin for all members.

Examples of clades include Vertebrata (vertebrates), Mammalia (mammals), Aves (birds), Angiospermae (flowering plants), and Arthropoda (arthropods). Recognizing clades helps biologists organize diversity according to evolutionary history, compare lineage-specific traits, and trace the branching events that shaped present-day life.