Taxonomic Swap 83338
(Committed on 2022-04-02)
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Comments
Thank you, @whiteoak. This will stay in draft.
The short version is that changes in generic circumscription based on a finding that a previously recognized genus is paraphyletic (or its recognition renders another genus paraphyletic) molecular phylogeny often result in two possible and 'opposite' solutions: you can split additionally to have finer-level monophyletic genera, or you can lump everything up to have one large monophyletic genus. Astonishingly, there is no existing good guidance on which course to take, and different investigators use very different rationales: which course will cause the fewest new combinations, which course will upset "the user community" the least, which is easiest, the degree of confidence and solidity of the phylogenetic tree and if it is likely to change with further or different sampling, whether the newly circumscribed genus or genera "makes intuitive sense", whether it or they are practically "diagnosable", the details of the tree topology, etc., etc.
It's worth noticing that many of these "rationales" have an implicit "lumping" bias -- lumping is also less risky because fewer distinctions need to be made. If yo have a choice between recognizing 4 small genera or one large one that includes all 4, it's easier intellectually and riskfree that you might be contradicted in the future to just lump. That doesn't mean that it's the right choice.
In this case, Polygonella is a monophyletic clade, very distinctive biogeographically, morphologically, easy to diagnose, etc. Continuing to recognize it makes a ton of sense by almost all rationales mentioned above and then some -- but it is a split that would require some minor rearrangements in the rest of the broader group, such as an expansion of Duravia with a few new combinations. Exactly how many taxa (and which) need to be moved from Polygonum to Duravia (this does NOT involve Polygonella) is not absolutely clear. In my strongly held opinion, we are better to allow some constructive ambiguity while that is being resolved, than to overthrow the genus Polygonella out of expediency.
as @whiteoak said, this will likely go back to Polygonella as a monophyletic genus at some point once the necessary combinations in Duravia and other genera if needed are made. However, since this is a 1 to 1 synonym and not a lump or a split at the species level it is not critical for iNaturalist which we use, as long as there is only one name used at a time for all records of this species (and the same for the several other species of Polygonella that are affected by the subsuming of the genus Polygonella into Polygonum)
I’ll be maintaining Polygonella in the Southeastern Flora (2020).