Phenetics

Phenetics is the first of these two practices, and its principle is to compare many anatomical traits in an attempt to deduce taxonomic affinities based on measurable similarities and differences. Phenetics does not however, attempt to ascertain any phylogenetic relationships. Assigning phylogenies is left up to the pheneticist, and may therefore be largely subjective. However, it is argued that if there are enough characters included then any analogies will be swamped by the degree of homology (Paterson 1997). Phylogenies are based on overall morphological similarity, and to calculate the degree of similarity between species, or operational taxonomic units (OTU's) as they are termed in phenetics, a character matrix is drawn up. From the matrix a table of similarities (phenogram) can be calculated, which indicates patterns of similarities between species.

The pheneticist is then free to postulate branching sequences, which might result in the observed relations. Rather than deducing strict phylogenetic reconstruction, phenetics assumes a uniform rate of mutation within a tree and therefore estimates a measure of evolutionary time between speciations. Therefore, instead of identifying branch points of a tree, it offers a measure of branch length.

One important problem of phenetics is that animals evolve and adapt constantly. Some of these changes result in speciation, while others simply modify the characteristics of the whole population. Phenetics fails to distinguish between these two types of change. Cladistics, however identifies only the events which result in speciation.