Phenetics

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In biology, phenetics, also known as numerical taxonomy, is an attempt to classify organisms based on overall similarity, usually in morphology or other observable traits, regardless of their phylogeny or evolutionary relation.

Phenetics has largely been superseded by cladistics for research into evolutionary relationships among species. However, some biologists continue to use certain phenetic methods, such as neighbor-joining, as a reasonable approximation of phylogeny when cladistic methods are too computationally expensive.

Many systematists continue to use phenetic methods, particularly in addressing species-level questions. While the ultimate goal of taxonomy includes finding the 'tree of life' - the evolutionary path connecting all species - field taxonomists also need to be able to separate one species from another. Classifying groups of diverse organisms that differ by very subtle differences is difficult using a cladistic approach. Phenetics provides numerical tools for examining overall patterns of variation, allowing researchers to identify discrete groups that can be classified as species.

Phenetic techniques include various forms of clustering and ordination. These are sophisticated ways of reducing the variation displayed by organisms to a manageable level. In practice this means measuring dozens of variables, and then presenting them as two or three dimensional graphs. Much of the technical challenge in phenetics revolves around balancing the loss of information in such a reduction against the ease of interpreting the resulting graphs. Modern applications of phenetics are common in botany, and you'll find some examples in most issues of the journal Systematic Botany.

It is interesting to note that many of the techniques developed by phenetic taxonomists have been adopted and extended by community ecologists.

Phenetics doesn't provide any information about the evolutionary relationships among species, but there is no reason that species identified using phenetics cannot subsequently be subjected to cladistic analysis. Traditionally there has been a great deal of heated debate between pheneticists and cladists, but the two perspectives need not be mutually exclusive. Phenetics is a powerful tool, but it has limitations. The same is true of cladistics. Both have their place in systematics.

[edit] References

Phenetics was developed by many people, but the most influential are Sneath and Sokal. Their book is still the primary reference for this sub-discipline, although it is now somewhat dated and out of print.

Sneath, P. H. A. & R. R. Sokal. 1973. Numerical taxonomy — The principles and practice of numerical classification. W. H. Freeman, San Francisco. xv + 573 p.

An excellent, recent textbook on numerical techniques used by ecologists and taxonomists is Legendre and Legendre:

Legendre, Pierre & Louis Legendre. 1998. Numerical ecology. 2nd English edition. Elsevier Science BV, Amsterdam. xv + 853 pages.

Phenetics should not be confused with phonetics, the study of speech sounds, despite the similarity in pronunciation.