> Well, what criteria (if any) do they use? How (if at all) do they > define "genus"? > > After all, the ICZN gives us full "taxonomic freedom" in this > respect. You'd have to ask them that. All I am saying is that those people are out there, there are a lot of them, and (since they tend to be old-timers) they are often in positions of authority and influence. That doesn't (of course) mean that you have to agree with them; but it does mean that you would be wise to be aware of their existence and their opinions.
I am aware of pieces of doublethink like this one, if that's what you mean: http://app.pan.pl/archive/published/app52/app52-651.pdf
I'm even aware of the attempt to create a "Biological Genus Concept" in extrapolation from the two Biological Species Concepts (if there are or can be viable hybrids, we're dealing with a single genus), but only something like three people have ever advocated it, let alone used it in anything near a systematic way.
And perhaps not to present your own as though they represent a position that is universally held.
It's not just my own, it's the ICZN's: if you don't like an existing classification, you're completely free to publish your own based on any reasons or lack thereof you like, and neither is ever wrong.