Klade

A klade is a group in evolutionary biology that includes a common ancestor and all its descendants, forming a single branch on the evolutionary tree. The term derives from Greek klados meaning "branch." In cladistics, a klade is defined by monophyly: all members share a last common ancestor not shared with any organisms outside the group. Clades are diagnosed by synapomorphies—shared derived characteristics—or by genetic data that resolves their relationships in phylogenetic trees.

Clades are contrasted with grades and with polyphyletic groupings. A grade groups organisms based on similarity or level of complexity but not necessarily all descendants; a polyphyletic group collects organisms from different ancestors. A paraphyletic group includes a common ancestor and some, but not all, descendants.

Methods: Cladistic analysis uses morphological and molecular data to construct phylogenetic trees and identify clades. Clades are nested; a larger clade contains one or more smaller nested clades. For example, Vertebrata (which includes mammals, birds, reptiles, amphibians, and fish) is a widely recognized clade. Modern plant and animal taxonomy often expresses classifications in terms of clades to reflect evolutionary relationships.

History: The concept was formalized in cladistics by Willi Hennig in the mid-20th century, which popularized the use of monophyly as a criterion for taxonomic validity.

Impact: The klade concept underpins contemporary approaches to taxonomy, systematics, and conservation, shaping how scientists define biodiversity and evolutionary history.