cladistics

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cladistics A method of classification in which animals and plants are placed into taxonomic groups called clades strictly according to their evolutionary relationships. These relationships are deduced on the basis of certain shared homologous characters (known as synapomorphies; see apomorphy) that are thought to indicate common ancestry (see monophyletic). Implicit in this is the assumption that two new species are formed suddenly, by splitting from a common ancestor, and not by gradual evolutionary change. Also, it requires that truly homologous characters are distinguished from homoplasic features, i.e. ones resulting from convergent evolution (see homoplasy). A diagram indicating these relationships (called a cladogram) therefore consists of a system of dichotomous branches: each point of branching represents divergence from a common ancestor, as shown in the diagram. Thus the species A to F form a clade as they share the common ancestor X, and species A to D form a clade of a different taxonomic rank, sharing the ancestor X₂. Species C to F do not form a clade, since the latter must include all the descendants of a common ancestor.