Clade - Clades and Phylogenetic Trees

Clades and Phylogenetic Trees

The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics, the latter term being derived from "clade" by Ernst Mayr (1965). The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms; they, and all their branches, are phylogenetic hypotheses.

Three methods of defining clades are featured in phylogenetic nomenclature: node-, stem-, and apomorphy-based:

• In node-based definition, clade name A refers to the least inclusive clade containing taxa (or specimens) X, Y, etc. A is the clade consisting of the last common ancestor of these taxa together with all the descendents of that ancestor. The ancestor is the branch point, or node.
• In stem-based definition, A refers to the most inclusive clade containing X, Y, etc., and their common ancestor, down to where Z branches off below A. Taxa are included between the node of A and down to (but not including) the branching point to Z; that is, the stem of A.
• In apomorphy-based definition, A refers to the clade identified by an apomorphy (a trait) found in X, Y, etc., and their common ancestor.