Phenetics

From Palaeos.org

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

Phenetics is an approach to grouping organisms based on total (or "raw") similarity. Although its history dates back centuries to the French botanist Michel Adanson, phenetics underwent something of a renaissance in the 60's, 70's and early 80's in response to a growing dissatisfaction with what its practitioners viewed as the arbitrary and nonquantative approaches that rose to prominence in the 1950's. Particularly troubling was the evolutionary taxonomy of Ernst Mayr, George Gaylord Simpson and others (described more fully below) that appeared to be more "art" than science.

On this basis, rigorous computationally-driven clustering methods were developed to combat these perceived problems. Information about organisms would be gathered, fed into computers, and out would come hierarchical arrangements of organisms based on overall similarity, typically arranged in a "tree" of sorts called a phenogram.

Phenetics has largely been superseded by cladistics for research into evolutionary relationships among species.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 important to draw a distinction between phenetics as an approach to taxonomy, and phenetics as a tool for deciphering the evolutionary relationships of organisms. Although phenetic clustering can and has been used to generate phylogenetic trees, to the phenetic taxonomist, any convergence of his phenogram on a phylogentic tree is purely coincidental. Even if the groups he were to arrive at phenetically were nothing like the groups we'd discover if we had a chance to look at the true tree of life, it wouldn't matter; there are reasons, they think, for representing living things this way independent of evolution.

Although phenetics doesn't provide any information about the evolutionary relationships among species, 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.

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.

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