Box 16.1Phenetics and cladistics

Phenetics, when first introduced (Michener and Sokal, 1957), challenged the prevailing view that classifications should be based on comparisons between a limited number of characters that taxonomists believed to be important for one reason or another. Pheneticists argued that classifications should encompass as many variable characters as possible, these characters being scored numerically and analyzed by rigorous mathematical methods.

Cladistics (Hennig, 1966) also emphasizes the need for large datasets but differs from phenetics in that it does not give equal weight to all characters. The argument is that in order to infer the branching order in a phylogeny it is necessary to distinguish those characters that provide a good indication of evolutionary relationships from other characters that might be misleading. This might appear to take us back to the pre-phenetic approach but cladistics is much less subjective: rather than making assumptions about which characters are ‘important’, cladistics demands that the evolutionary relevance of individual characters be defined. In particular, errors in the branching pattern within a phylogeny are minimized by recognizing two types of anomalous data.

  • Convergent evolution or homoplasy occurs when the same character state evolves in two separate lineages. For example, both birds and bats possess wings, but bats are more closely related to wingless mammals than they are to birds (see Figure A). The character state ‘possession of wings’ is therefore misleading in the context of vertebrate phylogeny.
  • Ancestral character states must be distinguished from derived character states. An ancestral (or plesiomorphic) character state is one possessed by a remote common ancestor of a group of organisms, an example being five toes in vertebrates. A derived (or apomorphic) character state is one that evolved from the ancestral state in a more recent common ancestor, and so is seen in only a subset of the species in the group being studied. Among vertebrates, the possession of a single toe, as displayed by modern horses, is a derived character state (see Figure B). If we did not realize this then we might conclude that humans are more closely related to lizards, which like us have five toes, rather than to horses.

Phenetics and cladistics have had an uneasy relationship over the last 40 years. Most of today's evolutionary biologists favor cladistics, even though a strictly cladistic approach throws up some apparently counter-intuitive results, a notable example being the conclusion that the birds should not have their own class (Aves) but be included among the reptiles.

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From: Chapter 16, Molecular Phylogenetics

Cover of Genomes
Genomes. 2nd edition.
Brown TA.
Oxford: Wiley-Liss; 2002.
Copyright © 2002, Garland Science.

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