Plea for chronotaxonomics

With respect to conveying useful comparative information, current biological classifications are seriously flawed because they fail to (i) standardize criteria for taxonomic ranking and (ii) equilibrate assignments of taxonomic rank across disparate kinds of organisms. In principle, these problems could be rectified by adopting a universal taxonomic yardstick based on absolute dates of the nodes in evolutionary trees.” (John C. Avise & Glenn C. Johns, 1999, p.7358)


Background

A phylogenetic tree represents a graphical illustration of evolutionary relationships among taxa. The assignment of categorical ranks for classification, however, is not an inherent attribute of a phylogenetic tree but is based on downstream subjective decisions. 

The ICZN (International Commission on Zoological Nomenclature), instead of providing guidelines for orientation, proudly claims to ensure the freedom of scientists to classify animals according to their personal philosophy. But what is the benefit of regulating the naming of taxa without properly defining them? 

Dissatisfied with the existing ICZN code, a minority of taxonomists suggested to abolish categorical ranks altogether (e.g. De Queiroz & Gauthier, 1999). Later the critics developed a rank-free alternative taxonomic system, the PhyloCode (De Queiroz & Cantino, 2020). 

Chronotaxonomics

Personally, I favour the idea of Avise & Johns (1999) to maintain categorical ranks provided that their assignment is strictly tied to taxon age as reflected by the number of genetic differences. Other features like reproductive isolation, phenotype, niche differentiation, and vocalisation should not be considered. 

Alternative molecular taxonomics

All approaches that exclusively rely on molecular data to infer taxonomic classifications, including species delimitation, underlie the same basic principle. The great advantage of chronotaxonomics lies in the fact that it is more plausible and intuitive to set temporal thresholds than genetic thresholds. However, the immediate impact of fossil calibrations on chronotaxonomics is often negligible due to the prevalent lack of relevant fossils. 

References

Avise JC, and Johns GC (1999), Proposal for a standardized temporal scheme of biological classification for extant species, Proc. Natl. Acad. Sci. USA 96, 7358-63. (pdf)

De Queiroz K, and Gauthier J (1992), Phylogenetic taxonomy, Annu. Rev. Ecol. Syst. 23, 449-480. (link) 

De Queiroz K, and Cantino P (2020), International Code of Phylogenetic Nomenclature (PhyloCode), 190 pages. CRC Press. Boca Raton, FL. (free online access)