Determining temporal threshold values

Primary ranks (species and genera) need to be defined by temporal thresholds to objectively assign species status and genus affiliation, respectively. Secondary and tertiary ranks (subspecies and infrasubspecies) must also be taken into account, since species often exhibit substantial intraspecific diversification. Infrasubspecies are not regulated by the ICZN-Code, but article 45.5 states that "a fourth name published as an addition to a trinomen automatically denotes an infrasubspecific entity".

To delimit taxa, a continuous series of ever-splitting lineages has to be divided into individual sections, i.e. taxonomic ranks. However, in the search for suitable threshold values (cutoffs), utmost caution is required to ensure that existing taxon compositions are preserved as far as possible to minimise nomenclatural disruptions. To guarantee overall nomenclatural stability, a wealth of taxonomic information must be compiled, evaluated and implemented. Predictably, it will take a long time until taxonomists will agree on adopting temporal threshold values (cutoffs) to delimit categorial ranks. 

Species status might be attributed to taxa that are at least two million years old (following Avise et al., 1998; Price, 2008, and Hudson & Price, 2014). Likewise, Alström et al. (2018b, Fig.2) included a two-million-years cutoff in their locustellid timetree to distinguish intra- from interspecific divergences. 

Based on the evaluation of numerous papers on avian phylogeography and systematics, I propose temporal threshold values (cutoffs) to delimit species, subspecies and infrasubspecies, in the following way: 

• species rank: 2 Ma
• subspecies rank: 1 Ma
• infrasubspecies rank: 0.5 Ma

Exemplary infrasubspecies-level timetree depicting the relationships among eight extant taxa whose ranks have been delimited by the temporal threshold values that are favoured here. Note that infrasubspecific names are put in brackets to distinguish them from subspecific names. 

 

Comment

When long-established genera finally turn out to be non-monophyletic, taxonomists are left alone with the decision to either break up large non-monophyletic genera into several smaller genera (thus proliferating new genus names), or to merge several non-monophyletic genera to one large genus (thus eliminating familiar genus names). Both camps, the “lumpers” and the “splitters”, should be considered victims of taxonomic codes that refuse to provide clear guidelines for delimitation. 

Additional problems are encountered on the species-level rank. Shall newly discovered low-level taxa be treated as species, subspecies, or infrasubspecies? Again, there are no guidelines. Instead, the ICZN-Code proudly claims to ensure the freedom of scientists to classify animals according to taxonomic judgements. But what is the benefit of regulating the naming of taxa without defining them? Isn’t that taking the second step without the first?

It is to the great credit of Kevin de Queiroz (2007, 2011) that he disclosed several shortcomings of current taxonomy. However, Kevin de Queiroz himself failed at providing a sound alternative. His statement (de Queiroz, 2011, p. 28) that „species are simply separately evolving lineages at the inclusive end of the population level continuum“ has two weaknesses: first, species are taxa, not lineages, and secondly, species have to be unequivocally distinguished from close sister species and infraspecific taxa, respectively. The phrase "inclusive end of the population level continuum” is deliberately kept vague and needs to be specified to become meaningful. In its present form, the statement merely corresponds to a species concept that is still devoid of an accompanying species-delimitation approach. Likewise the PhyloCode, developed by Kevin de Queiroz and colleagues, is no step forward, since it depends upon the traditional Codes and their Types, and does not solve the species problem (Brower, 2020). 

In view of the general discontent with existing approaches of delimiting individual species, a number of theoretical proposals and software tools for computer-assisted species delimitation have been put forward to address the "species problem", e.g.:

• Genealogical sorting index (Cummings et al., 2008)
• Genealogical divergence index (Jackson et al., 2017)
• Multispecies coalescent model (Rannala & Yang, 2003, 2017)

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