Contributors to this list have asked if the “the requirement that genera be monophyletic” increases or decreases the number of genera. It does neither. If a tribe or subfamily is monophyletic, you could adopt a “lumper” view and merge the whole lot into just one genus. On the other hand, a “splitter” might clip the phylogenetic tree up into very small monophyletic twigs with one or a few species in each and give each one a generic name.
It is important to realize the distinction between taxonomy (= classification) and nomenclature when discussing name problems. Nomenclature is practised according to internationally acknowledged rules based on principles, some of which date back to Linnaeus. The most important rule is that all taxa must have types, which serve as anchor points for names. Names are given to types for taxa in certain categories (genus, tribe, family etc.) and have forms according to category. A name of a species is a binomial, consisting of a generic name plus a specific epithet. This very unfortunate (i.m.h.o.) principle was established by Linnaeus himself. He considered genera as natural entities that could be discovered and had no idea of how many generic names we would have to deal with 200 years later - he recognized only eight orchid genera!
It happens that names must be abandoned for not conforming to nomenclatural rules. An old name may be found to have been published without a description or a proper citation of one, or two different generic names may be shown to be based on the same type. The first published name then has to be adopted even if it has been much less used. There is nothing to do about it and you have better change your labels to the correct name at once. Example: When Lindley established the new genus Haemaria in 1840 with Goodyera discolor as type, he overloked (or ignored!) that A. Richard already had made G. discolor type of the generic name Ludisia in 1825.This was first realized in 1970, but there was nothing to do but change the labels.
Classification, on the other hand, has no rules. Anybody can suggest that a certain set of species constitute a genus, a tribe, a family, superorder or whatever. You can publish a paper claiming that all orchids with hairy stems and red flowers belong to one genus. If this set includes a species that already is a type for a name at the generic level, you must use this name for the whole lot, even if it originally was intended for a very differently defined set of species. If there is more then one name available, you must use the first one validly published. If none of the hairy-stemmed and red-flowered species are generic types already, you can go on and erect a new generic name based on a type-species of your own choice.
Are there really no rules? Nope, taxonomy is absolute anarchy. However, if you suggest something really weird you may not get your paper accepted for publication in a mainstream scientific journal. You can then opt for a more obscure journal or print your own book with all your new taxa and names! The compilers of the Index Kewensis and the International Plant Name Index will have to include them, but they may be largely ignored. Nobody has to accept name changes caused by new ideas on classification; you can keep your old labels. Example: If you prefer Bulbophyllum in a broad sense you can safely disregard all suggested split-off segregates like Ferruminaria, Mastigion, Rhytionanthus and even Cirrhopetalum. And you don't necessarily have to agree with J.J. Smith (1933) that Doritis Lindley (1833) should be regarded as part of Phalaenopsis Blume (1825).
So there is really no formal requirements like “taxa should be monophyletic”. Very many taxonomists (I am one!) think they should, but a substantial minority prefers more relaxed classifications, sometimes emphasizing similarity, sometimes assumed phylogeny. The next issue of the journal Taxon will include an appeal from >100 recognized plant taxonomists who reject exactly this “requirement”. Of course, there will be a rebuttal from other taxonomists in the next issue after that one. But there will never be formal rules on principles of classification.
Moreover, even among taxonomists who agree on certain classification principles there will be arguments about how to use data. Measures of similarity can be extracted in many different ways from molecular data, and there are several conflicting ideas about how to estimate phylogeny from these data.
What if you don’t care about all this scientific stuff and just want an unambiguous name for some plant or animal? Be conservative - use established names in a sense already used in standard works. Wait and see if new taxa become recognized in standard reference works. You can always add “As in this-widespread-handbook” on labels, and nobody can claim that your label is “wrong”.
Excuse me for this long (845 words) and perhaps boringly non-flammable posting. I hope it may help to explain why using names for plants (and animals) is such a confusing business. - Finn N Rasmussen (taxonomist), Copenhagen.
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