Transformed cladistics

Transformed cladistics, also known as pattern cladistics is a proposed classification system within cladistics which excludes common ancestry from cladogram analysis. It was popularized by Colin Patterson in the 1980s, but has few modern proponents.

The standard approach to cladistics which traces back to Willi Hennig (1950) groups together organisms based on whether or not they share characters or character states that derived from a common ancestor. Transformed cladists instead maintain that cladistics should be free from the assumption of common descent or the theory of evolution (as a process) altogether, and based only on empirical data:

"If classifications (that is, our knowledge of patterns) are ever to provide an adequate test of theories of evolutionary processes their construction must be independent of any particular theory of process." (Platnick, 1979)

In other words, pattern cladists argue that the less information a classification presupposes, the fewer errors creep in, and greater objectivity results. They draw a distinction between patterns, which are observed, and processes, which may be inferred from patterns, but which should not be presupposed. Joseph Henry Woodger before the emergence of transformed cladistics as a school criticized phylogenetic systematics on the grounds that homology by way of common ancestry is "putting the cart before the horse, because descent from a common ancestor is something assumed, not observed. It belongs to theory, whereas morphological correspondence is observed." (Woodger, 1945). Colin Patterson later wrote similarly:

"We must remember the distinction between the cart--the explanation--and the horse--the data. And where models are introduced in phylogenetic reconstruction, we should prefer models dictated by features of the data to models derived from explanatory theories." (Patterson, 1994)

Pattern cladists, like standard cladists, limit their classifications to nested sets (patterns) of synapomorphies, but they argue that the characters are irrespective to common ancestry:

"[T]o state a cladogram is a synapomorphy scheme invites the rejoinder that a cladogram must, therefore be a phyletic concept. Not so, for by ‘synapomorphy’ we mean ‘defining character’ of an inclusive taxon." (Nelson & Platnick, 1981)

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