Recall that monophyletic taxa are complete branches, or clades, of a phylogenetic tree, beginning with the founding ancestral species and including all the descendant species of the common ancestor (all twigs of the branch). According to the prevailing practice of phylogenetic taxonomy, a clade-taxon at one taxonomic rank (e.g. family) can be subdivided into smaller clade-taxa (e.g. genera) using the same criteria, but none of the subclades can be given the same rank as the main clade (e.g. there cannot be a family within a family). That is why birds and reptiles cannot both be ranked as formal classes, as was done traditionally.
|As we envision the process of evolution, clades
branch to beget new clades. Successful
clades further branch into bushy clusters of
species that we recognize as genera.
Traditional taxonomists would view the four groups as a succession of distinct genera, one on top of the other. They would see nothing wrong with new genera arising from older genera in sequence over time – it is necessary in fact, if we are to simultaneously believe in evolution and have a system of classification.
Phylogenetic taxonomists view the diagram differently. A clade is a clade, from the founding species through all of its descendants. Subclades are viewed as nested within the main clade, rather than emerging on top of them. If all four were considered genera, both A and C would be paraphyletic, because some parts have been removed. "A" can be considered a monophyletic genus, only if B, C, and D are included within it. If so, the latter 3 genera must be given lower rank than A (e.g. subgenera). Traditional taxonomists charge that this "lumping" solution would result in a collapse of the entire taxonomic system, for surely genus A itself evolved from some earlier genus, and so on back to the first genus of organisms.
Alternatively, B and D could be recognized as genera, but C and A would have to have higher rank (e.g. C as a subfamily that included genus D, and A as a family that included subfamily C and genus B). In that scenario, the remaining contents of A and C would have to be split into comparable subgenera and genera. However, "splitting" like that would just result in even more small, unclassifiable stem groups, and not really solve the problem.
Yet monophyletic taxa are the foundation of phylogenetic taxonomy. Is there no way out of this dilemma?
The phylocode makes a lot of sense, and in practice, we don’t worry so much about ranking the bigger clades of life any more. We talk of Magnolids, Eudicots, Monocots, Amborellids, etc. in discussions of the evolution of flowering plants, but it seems that no one is seriously trying to squeeze them into classes, subclasses, etc. any more. There seems to be no point to it.
It would appear that the prospects for monophyletic-only genera destabilize and collapse the more information we have about ancestral organisms. But does it really? Perhaps all this discussion of ancestors having names, and genera evolving from genera, is putting the cart before the horse.
The determination of generic-level characters is a subjective judgement that in this context inevitable. Otherwise, how do we decide that our own genus, Homo, is in fact a genus, not just a section of Australopithecus? Anthropologists (some at least) have emphasized the "minimization of paraphyly" by lowering the threshold for generic characters. Other systematists might choose to minimize the proliferation of small, barely-distinguishable genera by raising that threshold. I have argued that minimal ancestral genera, such as Praeanthropus, are not taxonomically paraphyletic, as they are not identified as ancestral in the cladistic process. How broadly or narrowly such ancestral genera are defined becomes an issue of how much difference should be required to distinguish genera from one another. This issue is far less cataclismic than the black-and-white battle between pro- and anti-paraphyletic forces that has unnecessarily preoccupied systematists for so many decades.
I would not necessarily extend these arguments to higher levels of classification, to talk about “class-worthy” characters, or “phylum-worthy” characters, for example. It is not necessary to go there. It may be that un-ranked clade names are a better option at those levels. Genera are uniquely important in the taxonomic hierarchy, however, as they are necessary for naming species by the binomial convention, so we must do what we can to maintain their universal application for all appropriately characterized clusters of species, be they current or ancestral.