In biology, phenetics, also known as numerical taxonomy or taximetrics, 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Phenetics has largely been superseded by cladistics for research into evolutionary relationships among species.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, certain phenetic methods, such as neighbor-joining, have found their way into cladistics, as a reasonable approximation of phylogeny when more advanced methods (such as Bayesian inference) are too computationally expensive.
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.
Consider for example songbirds. These can be divided into two groups - Corvida, which retains ancient characters in phenotype and genotype, and Passerida, which has more modern traits. But only the latter are a group of closest relatives; the former are numerous independent and ancient lineages which are about as distantly related to each other as each of them is to the more modern songbirds. In a phenetic analysis, the large degree of overall similarity found among the former will make them appear to be monophyletic too, but their shared traits were present in the ancestors of all songbirds. It is the loss of these ancestral traits rather than their presence that signifies which songbirds are more closely related to each other than to other songbirds.
But the two methodologies need not be mutually exclusive. In general, phenetics is today recognized to provide little if any information about the evolutionary relationships among taxa. But there is no reason why e.g. species identified using phenetics cannot subsequently be subjected to cladistic analysis, to determine its evolutionary relationships.
Phenetic methods can be superior to cladistics when only the distinctness of related taxa is important, as the computational requirements are lower. On the other hand, whenever information on the evolutionary history of taxa is needed for a study, cladistic methods are used today.
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 - in fieldwork one needs to be able to separate one taxon from another. Classifying diverse groups of closely-related 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.
Modern applications of phenetics are common in botany, and some examples can be found in most issues of the journal Systematic Botany. Indeed, due to the effects of horizontal gene transfer, polyploid complexes and other peculiarities of plant genomics, phenetic techniques in botany - though less informative altogether - are also less prone to errors compared cladistic analysis of DNA sequences.
In addition, many of the techniques developed by phenetic taxonomists have been adopted and extended by community ecologists, due to a similar need to deal with large amounts of data.
An excellent, recent textbook on numerical techniques used by ecologists and taxonomists is Legendre and Legendre: