A few remarks on CKing’s latest…

First the appeal to inclusivity and tolerance, including the remarkable statement that pheneticists are entitled to their beliefs. Anybody who still thinks that grouping by similarity is the best method of inferring phylogeny (tree-building) is simply mistaken in their belief. Tolerance can be taken too far.

Hijacking of terms: I have come across the term ‘holophyletic’ and the sense of ‘monophyletic’ that includes paraphyly in one or two obscure places, but NEVER, that I recall, in an actual phylogenetic or taxonomic work from the last 30 years. I really don’t know the tradition they belong to; while it’s good to be able to read the dead classical languages, they are no longer a suitable medium for scientific discourse. Whether a hijacking was involved or not, the process of language evolution can hardly be reversed. As for “The term monophyletic has never meant groups that consist of a single ancestor and all of its descendants” – actually, that is exactly what it has meant since I first heard it (about 1980). HOW OLD ARE YOU, that the last few decades have passed without notice?

”Darwin has sometimes been misinterpreted as being a cladist in his classificatory philosophy. Nothing can be further from the truth.” That last bit is an exaggeration, you must admit; many things further from cladism can be imagined (e.g. cheerful acceptance of polyphyly, defining groups by their ‘destiny’ rather than their content or characters, creationism). Cladistic classification is ‘pure’ grouping by genealogy, whereas Darwin believed (as you quote) that “the arrangement of the groups within each class, in due subordination and relation to each other, must be strictly genealogical in order to be natural” (sounds very Hennigian so far), but also that different degrees of modification could usefully be expressed by (to simplify his circumlocution) sometimes using different Linnean ranks for sister taxa, or assigning ancestors and descendants to separate groups of the same rank.

Well, Darwin was a great biologist – a revolutionary, saint and prophet of science indeed – but only came close, without ever explicitly stating or applying the principles of homology and synapomorphy that turned phylogenetics into a science. That all happened after his time (though it was happening well before Hennig got hold of the idea). Linneus’ great contribution had been the hierarchical system of classification (as opposed to the eclectic lists or weird numerological/astrological arrangements that went before); Darwin’s advance in systematics (i.e. apart from Natural Selection) was the principle that a natural, genealogical classification is possible and desirable, and would be strictly hierarchical as a direct consequence of descent with modification (so the manifest success and ‘naturalness’ of Linneus’ system was confirmation for Darwin’s theory). Hennig explicitly proposed that a natural classification should use only monophyletic taxa (i.e. only clades should be named). Traditionalists like CKing find this objectionable, but that does not give them exclusive rights to be called Darwinians. Most of the people who make and use classifications these days do prefer (or expect) taxa to be monophyletic, but that does not make us ‘Hennigians’ in any useful sense because most of his concepts, methods and terms have been superseded; modern systematists are much more likely to read, cite and learn from Darwin’s timeless classics than from Hennig.

In Darwin’s day, importing arbitrary judgements about what degree of divergence justifies a certain ‘rank’ was unavoidable. Hennig kept ranks, and thought they should correspond to the age of clades (genera are so many millions of years old, families somewhat older, etc.). These days we have vastly more detailed phylogenetic information and are increasingly tossing out the idea that taxa of the same Linnean rank are equivalent in any meaningful way, so that while the standard endings of ‘family’, ‘order’ and other taxon names can still be useful for organizing lists and memory, many of us have decided to exclude such ranks from formal classification completely. When we mention Mammalia or Reptilia, we mean clades, not ‘Classes’; Caenophidia, Colubroidea, Elapidae and Hydrophiinae are a sequence of nested clades, and it means nothing real to (formally) recognize one of them as equivalent in rank to another taxon apart from its immediate sister group. Linnean binomina are an extremely handy mnemonic device that makes it possible for ordinary mortals to organize information on thousands of species in their heads, so I can’t see any net advantage in abandoning genera; but I like them monophyletic, and this is always possible except in the case of species that are actual ancestors of two or more genera that it would be impractical to lump. Since actual ancestors are unlikely to be observed (even as fossils) but may be indistinguishable from some of their descendants (like F – F14 in Darwin’s diagram), nothing is lost by naming a monotypic genus for the ‘ancestral’ species in such a case.

“If Chondropython is so highly derived, then it should be removed from the same genus as its ancestor, as Darwin did by removing A’s descendants from the genus which includes A. By doing so, we are showing that Chondropython is quite distinct morphologically, behaviorally and ecologically than Morelia.” What such a split actually does is to obscure the fundamental similarity (homology) of viridis with other members of the Morelia spilota group. It has diverged in some ways, but on average (looking beyond superficial appearance, and using the tree and character data that we now have) far less than M. spilota has from M. amethistina. It is false to say that clades “tell us nothing about the morphology, ecology and behavior”: as Darwin said, “community of descent the one known cause of close similarity in organic beings”, and it is therefore the best predictor of similarity in characters that have yet to be studied.

With no formal ranks above the genus, and no taxa at any level (above species) that are provably non-monophyletic, we can have a workable but streamlined system. Some may prefer classifications with a lovely baroque feel - taxonomic equivalents of festoons, curlicues and little plaster cherubs - but I can’t see the point.
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John D. Scanlon
Riversleigh Fossil Centre
Outback at Isa
Mount Isa, Queensland, Australia

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