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Phenetics, or Phenetic systematics, is not the same as cladistics, but we've placed it here basically because there's nowhere else to put it. It is no longer a distinct school of systematic biology, and has at best limited application. Nevertheless, like cladistics it began as an alternative to evolutionary systematics, and it pioneered the sort of quantitative computational phylogeny and quantative statistical analysis that would later become central to both cladistics and molecular phylogeny. There was even a short-lived school of cladistics, called Pattern cladism, that was very similar to phenetics.
Phenetics started out as a school of numerical taxonomy that was developed in the late 1950s by bacteriologist Peter H. Sneath, entomologist Charles D. Michener, and quantitative geneticist Robert R. Sokal, that classifies organisms on the basis of overall morphological or genetic similarity. It is not the same as cladistics, but it is a prequel to it, sharing many features in common. The school of Pattern Cladism could be considered a synthesis of cladistics and phenetics
Phenetics mainly involves observable similarities and differences irrespective of whether or not the organisms are related. It involves grouping types together in clusters; types with many close relatives would be in a cluster. The development of Phenetics, which was intended to replace evolutionary systematics, was inspired through the quantitative successes and advances in genetics (e.g. discovery of DNA by Watson & Crick (1953)), chemistry and physics, on the other hand as a reaction to positivism and incorporation of a strictly pragmatic approach, which denies that we can know the thing in itself (hence the rejection of phylogeny and evolution). The availability of computers (at this time still big hulking things) also facilitated and encouraged quantitative data comparisons. It uses distance matrix-based methods to construct trees based on overall similarity, which is often assumed to approximate phylogenetic relationships.
Phenetics classifies organisms on overall similarity, usually in morphology or other observable traits, regardless of their evolutionary relationship. It stressed the use of many unweighted characters assessed by overall similarity, purging all intuition and subjectivity and striving to be theory neutral, objective, and quantitative, with observation, description and ordering performed as precisely, objectively and repeatably as possible. Hence all evolutionary and phylogenetic interpretations are rejected as too difficult and subjective. It was considered that phylogenetic reconstruction was nearly impossible to know with any degree of certainty, and therefore, if classification were to be scientific, this futile quest should be abandoned. (Stuessy 2009, UCMP)
In the end Phenetics was unsuccessful and eventually abandoned in favour of cladistics for a number of reasons, including numerous difficulties encountered owing to convergence (homoplasy, as individual characters assumed to be homologous were not carefully analysed), mosaic evolution, and a shortage of diagnostic characters (Mayr & Ashlock 1991, pp. 195-205). Even so, 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. (Wikipedia). Also, with the rise of molecular systematics, distance methods, which are basically phenetic methods, have become popular, although these are vulnerable to the same problems, especially that of homoplasy. (Mayr & Bock 2002 p.180). MAK130321
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