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Following is from wikpedia:

Under the traditional nomenclature codes, such as the International Code of Zoological Nomenclature and the International Code of Nomenclature for algae, fungi, and plants, taxa that are not explicitly associated with a rank cannot be formally named, because the application of a name to a taxon is based on both a type and a rank. The requirement for a rank is a major difference between traditional and phylogenetic nomenclature. It has several consequences: it limits the number of nested levels at which names can be applied; it causes the endings of names to change if a group has its rank changed, even if it has precisely the same members (i.e. the same circumscription); and it is logically inconsistent with all taxa being monophyletic.

Especially in recent decades (due to advances in phylogenetics), taxonomists have named many "nested" taxa (i.e. taxa which are contained inside other taxa). No system of nomenclature attempts to name every clade; this would be particularly difficult in traditional nomenclature since every named taxon must be given a lower rank than any named taxon in which it is nested, so the number of names that can be assigned in a nested set of taxa can be no greater than the number of generally recognized ranks. Gauthier et al. (1988)[6] suggested that, if Reptilia is assigned its traditional rank of class, then a phylogenetic classification has to assign the rank of genus to Aves.[7] In such a classification, all ~12,000 known species of extant and extinct birds would then have to be incorporated into this genus.

What is the precise meaning of "it is logically inconsistent with all taxa being monophyletic"?

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  • $\begingroup$ I've changed your title, removing the "please explain". This could be included in almost every title, and though it may seem polite, it is an impediment to readers finding out what question you are asking. $\endgroup$ – David Jan 21 '17 at 21:01
  • $\begingroup$ Maybe your should also put your question in the discussion page corresponding to the wikipedia article you cite. $\endgroup$ – bli Jan 23 '17 at 13:24
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You can find discussions of problems with rank-based taxonomy in light of phylogenetics in Ereshefsky 1994, de Queiroz 1996 and Ereshefsky 2002. To summarize, the main problems they identify are:

  1. It's hard to define what exactly an "order", "family" or "genus" is; in particular, taxa at the same rank aren't always comparable. For example, order Hymenoptera, which evolved 250 million years ago and consists of over 150,000 species, with order Carnivora, which evolved 42 million years ago and consists of "over 280 species".

  2. Not enough ranks: NCBI recognizes 29 higher taxa for Homo sapiens. That would require dozens of sub-and-super ranks to render into Linnaean taxonomy.

  3. Names based on ranks: current nomenclature allows certain ranks to be identified by their ending, for example names ending in "-ini" refer to tribes (e.g. Hominini, Canini). If one of these tribes turns out to be non-monophyletic, it may have to be moved to a higher or lower rank to keep its monophyly -- which would have to be renamed, increasing confusion.

  4. Redundant taxa: take division Ginkgophyta. It contains a single class, the delightfully named Ginkgoopsida, which contains a single order, Ginkgoales. In the Linnaean hierarchy, each of these taxa contains the next one along, but in terms of monophyly, they are exactly identical to each other, as they share exactly the same most recent ancestor.

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