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RE: Classification: A Definition

Jerry Harris wrote:

(Noting, of course, that "class" here is used in a philosophical sense, not a Linnean one.) I don't quite see how phylogenetic nomenclature -- well, phylogenetic systematics, that is, which is technically distinct from but inextricably linked to phylogenetic nomenclature (one is kind of useless without the other) -- doesn't meet these criteria. The nomenclature is a means of providing names to entities (classes) that exist/are ascertained to exist based on evolutionary descent; this differs from the Linnean system of providing names to entities based on a few gross physical similarities, sans regard to evolutionary relationships. Either way, it's using a set of criteria to designate (i.e., name) groups.

I would have said the same thing (though perhaps not as articulately as Jerry). At the end of the day, phylogenetic nomenclature does "classify" organisms, insofar as taxa are grouped by common descent to produce a hierarchy. _Tyrannosaurus_ is inside Tyrannosauridae, which is inside Coelurosauria, which is inside Theropoda, and so on... Similarly, _Passer_ is inside Passeridae, which is inside Passeriformes, which is inside Aves, which is inside Coelurosauria, and so on...

The major differences between the above and the Linnaean system is that (a) taxa are always grouped by common descent; and (b) a taxon's relative position in the hierarchy is not denoted by "ranks" (Kingdom, Phylum, Class, Order, etc). The Linnaean system would put _Passer_ and other birds in a separate "Class" (Class Aves) compared to _Tyrannosaurus_ and other non-avian theropods (Class Reptilia).

For the Monotremata example, Mammalia is a clade inside Synapsida, just as Monotremata is a clade inside Mammalia. Thus, by extension, Monotremata is a clade (one of many) inside Synapsida. The source of confusion for Anthony Docimo, as far as I can tell, is that he is assuming that the word "monotreme" is supplanted by "synapsid". This is conceptually incorrect.

As for the example of protists (which also surfaced on this list), the old "Protista" is the poster child for just how BAD the Linnaean system was (and still is). Not only did the "Protista" concept vastly understate the huge phylogenetic diversity of single-celled eukaryotes, but we even had a situation where certain protists were classified as BOTH animals and plants (e.g., _Euglena_). The latter was a product of grouping organisms based on "key features" or "gross physical similarities" (as Jerry puts it). Euglenids moved and fed like animals, but they also possessed chlorophyll like plants, therefore they got put into both Animalia and Plantae.



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