[Taxacom] Elimination of paraphyly: sensible or not?

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Thu Feb 8 16:07:55 CST 2018

Hi all,
I have been giving some thought to the cladistic obsession of eliminating paraphyly in taxonomic classification. For many taxa (above species), the subtaxa consist of one or more clearly monophyletic groups, plus a possibly paraphyletic residue (i.e. no apomorphies to bind the residue together into a monophylum). So, if we must eliminate paraphyly (or possible paraphyly), the only options are to either: (1) subsume the monophyletic subtaxa into the paraphyletic residue; or (2) break up the paraphyletic residue into monophyletic subtaxa. Effectively the two options may actually be equivalent. An example might help to illustrate my point. Let's take a simplistic view of reptiles as scaly tetrapods, birds as feathery winged bipeds derived from reptiles, and mammals as hairy tetrapods derived from reptiles. So, amniotes (reptiles, birds and mammals) are a monophyletic group, as are birds and also mammals, but not reptiles (reptiles being the "paraphyletic residue"). We wish to retain birds and also mammals as useful monophyletic taxa, for obvious reasons. So, what to do? Luckily, within reptiles there are some monophyletic subgroups of sufficient diversity to be useful, but this might not have been the case if all reptiles were just basically "skinks", with only species or perhaps also generic differences between them. Had this been so, amniotes would have to be taxonomically split between numerous (maybe hundreds) virtually identical taxa of "skinks", plus birds and also mammals as just two taxa at the same level (not necessarily a ranked level, but direct child taxa of amniotes). Would this be a useful classification of amniotes? I suggest that it would be far more useful to recognise a single paraphyletic taxon of reptiles (all the "skinks" in the hypothetical example), plus birds and also mammals (i.e. just 3 direct child taxa of amniotes). I wonder for plants, fungi and also invertebrates, if there might be many taxa analogous to the above hypothetical example, with a paraphyletic residue consisting of hundreds of "skinks", but also with just one or two very distinct and diverse monophyletic subtaxa? If so, would it be sensible to eliminate paraphyly or best just to live with a known paraphyletic residue as a unified subtaxon? Given the amount of limited resources which are being allocated to projects to eliminate paraphyly, to the detriment of alpha taxonomy, it would be nice to think that there was a clearly good reason for the elimination of paraphyly, but I'm not so sure that there is! The usual argument seems to be that you cannot make meaningful predictions from paraphyletic taxa, but how much biology does rely on the making of predictions based on taxon membership, and what proportion of those predictions end up being true anyway? For example, you might predict that a newly discovered braconid is a parasitoid, but a few braconids are phytophagous anyway. So, I guess that the main question that I am posing is whether we think that the benefits of monophyly justify the spending of so much limited resources on the elimination of paraphyly? Perhaps the elimination of paraphyly is being driven instead by economic factors, doing phylogenies being a more cost efficient way for institutional scientists to spend their time on than alpha taxonomy?

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