To amplify a point from earlier, where I said “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).”

… with another succinct statement by Darwin (Origin, Ch. XIV):

“In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, dimorphic forms, and acknowledged varieties of the same species, however much they may differ from each other in structure. If we extend the use of this element of descent – the one certainly known cause of similarity in organic beings, - we shall understand what is meant by the Natural System: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms, varieties, species, genera, families, orders, and classes.”

It is ambiguous in Darwin’s text whether ‘grades of acquired difference’ correspond in his thought to what we call ‘grades’ (typically paraphyletic if not polyphyletic in 20th century practice), or if he considered as distinct (and rejected?!) the idea of naming clades instead. I suspect it was just too soon for him to realize there was a difference; the concept of a clade and its diagnosis could be stated – e.g. “on the principle of inheritance, all the forms descended, for instance, from A, would have something in common” – but nobody had yet though of coining a rank-free term for “all the forms descended from A”.

Linnean classifications are hierarchical, consisting of ‘groups under groups’ as Mr D so often expressed it, but the traditional taxonomic hierarchy is typically not the same as the actual genealogical (phylogenetic) structure (which we can increasingly often determine exactly enough, hence the death of the ‘no-hope’ school of phenetics). Darwin belaboured the fact that there’s no criterion to regard hierarchy as real or natural except as a result of branching common descent, so why would he plump for a hierarchy that’s known to be contradictory to nature’s? I don’t think he ever meant to do so, though anti-cladists do it consciously every day.

Since Darwin’s day, users of classifications (for example, those designing comparisons and experiments to test evolutionary theory and particular patterns and processes) expect members of the same taxon to be ‘related’ to each other more closely than to those of other taxa at the same rank. Much of the time, accepted classification has failed to do what was expected, and much experimental effort has been wasted as a result. An example, probably typical: which species of Python should be compared serologically with Australian and New Guinea pythonines to test the systematic ideas of McDowell 1975? Schwaner and Dessauer 1981 picked the wrong one (P. regius) to test, precisely because McDowell had a paraphyletic concept of the genus but they assumed he meant it to be a clade.


CKing: “I agree, there is no reason to abandon the rank of genus. Neither is there any reason to abandon the other Linnaean ranks.”

Once the artificial and misleading features of paraphyletic taxa are recognized and we adopt a monophyletic arrangement instead, actually practicing systematics on real organisms (in contrast to bombinating in a vacuum ) will show the impracticality of ranks. For example, take Squamata (phylogenetic understanding of which has got quite detailed but not changed in broad outlines since Camp 1923):

All extant snakes belong to a taxon named Serpentes, traditionally called a suborder (sometimes an order). (There are also fossil snakes which – according to most of the evidence, and it should come as no surprise - lie outside the clade comprising all living snakes; clearly, whether to include them in Serpentes is a matter of arbitrary boundary fixing.) But snakes are (again, according to most of the evidence) derived from among the varanoid lizards; Varanoidea is traditionally called a superfamily within the infraorder Anguimorpha. And all anguimorphs are scleroglossans (… but I don’t know or care what the rank of that is supposed to be called…), and scleroglossans (together with Gekkota) are autarchoglossans, which along with Iguania make up Squamata (order, or subclass or whatever). All squamates that aren’t snakes would traditionally have been called ‘Lacertilia’ or ‘Sauria’ (oh no, that’s not right; I was forgetting mosasauroids and amphisbaenians, often given orders or suborders of their own…).

The actual (or likely) relationships can easily be expressed by a tree (cladogram), and having once seen such a tree (like the one in Camp’s 1923 monograph, or those of Estes et al. 1988), why would anyone go back to a confusing mess of redundant and meaningless terms (‘rank’ titles for higher taxa) and taxonomic concepts that falsify what we actually know? If we know (as we think we do) that snakes are varanoid, anguimorph, scleroglossan squamates, and only one of numerous independently evolved groups of limbless lizards, why ‘promote’ them to the same rank as ‘other lizards’? There is no imaginable justification apart from claiming ignorance of actual relationships – an interim solution at best, now requiring heroic effort to ignore the accumulated evidence.

Give up rank, or give up monophyly; the choice is easy. Thanks for the directions, Linneus, but we can see where we’re going now.
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John D. Scanlon
Riversleigh Fossil Centre
Outback at Isa
Mount Isa, Queensland, Australia

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