Clade - Nomenclature and Taxonomy

Nomenclature and Taxonomy

The idea of a clade did not exist in pre-Darwinian Linnaean taxonomy, which was based by necessity only on internal or external morphological similarities between organisms – although as it happens, many of the better known animal groups in Linnaeus' original Systema Naturae (notably among the vertebrate groups) do represent clades. The phenomenon of convergent evolution is however responsible for many cases where there are misleading similarities in the morphology of groups that evolved from different lineages.

With the publication of Darwin's theory of evolution in 1859, the concept of a "natural system" of taxonomy gained a theoretical basis, and the idea was born that groups used in a system of classification should represent branches on the evolutionary tree of life. In the century and a half since then, taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming, however, this principle is not always compatible with the traditional rank-based nomenclature. In the latter, only taxa associated with a rank can be named, yet there are not enough ranks to name a long series of nested clades; ranks determine the form of names, so names must in many cases change when a name is inserted into such a series; and taxon names cannot be defined in a way that guarantees them to refer to clades. For these and other reasons, phylogenetic nomenclature has been developed; it is still controversial.

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