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Classification and evolution - What are the three main schools of classification?


Cladistic classifications are monophyletic

Cladistic classifications only include monophyletic groups because only they have the unambiguous hierarchical arrangement of the phylogenetic tree. This is the theoretical beauty of cladism. Only monophyletic groups are formed in the cladistic conversion of a phylogenetic tree into a classification and nothing has to be known about the phenetic evolution of species within each branch. The figure opposite illustrates the simple relationship between cladistic classification and their phylogenetic tree.

The result of this is that cladistic classifications can be radical and counter-intuitive: for instance the category "fish" does not exist in cladism. Tetrapods evolved from lobe-finned fish (such as lungfish) and this means that the cow and the lungfish are grouped together because they share a more recent common ancestor than the lungfish and the salmon.

Cladism has other controversial features:

1. A group can change its name if it splits and one of the new lineages has hardly changed phenetically from its ancestor.

2. When a new group originates by hybridization between two species then the shape of the phylogeny does not imply a hierarchical classification.

Figure: there is a simple relationship between the phylogenetic (cladistic) classification of a group of species and their phylogenetic tree. (a) The phylogenetic tree of seven species. (b) Their cladistic classification. (c) The formal Linnaean classification, for species five as an example.

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