In taxonomy, the species rank refers to a low-level category which is hierarchically intermediate between the ranks of subspecies and genus. The species rank is not qualitatively different from other ranks. The term species refers to taxa that are defined on the basis of the species rank. Species are assigned a two-part name (e.g. Columba livia), with the first name representing the name of the affiliated genus. In nomenclature, the binomial species name is of particular importance, since it forms the basis for naming higher-level taxa. In order to assure a permanent primary source of reference, species are represented by a name-bearing type specimen or by a set of type specimens in scientific collections. 

Modern phylogenetic studies provide time-calibrated cladograms (chronograms, timetrees) that illustrate the natural relationships among organisms. However, phylogenetics per se cannot provide categorical ranks, because they are arbitrarily assigned to taxa. The lack of a natural “species signal” in phylogenetics implies that species cannot be detected but have to be defined. 

Assigning arbitrary ranks to taxa is a taxonomic endeavour that can, under objective criteria, only be accomplished by defining temporal thresholds. In fact, age should be the sole defining criterion for species delimitation (as discussed by Jarvis, 2016, and recommended for allopatric species by Price, 2008, and Hudson & Price, 2014). 

To establish reliable timetrees, differences in the DNA signature of taxa need to be quantified, before the observed differences can be translated into divergence times. Since this task is of paramount taxonomic importance, it will be detailed on a future separate page of this website. 

The theoretical concept of delimiting primary ranks (i.e. species and genera) via temporal thresholds is illustrated for an examplary timetree that comprises eight arbitrary taxa: 

Defining ranks by means of temporal thresholds is a straightforward way to comply with the mandatory requirement of the ICZN-Code to assign species status to taxa. Species that are delimited by temporal thresholds may be referred to as “chronospecies”. It should be noted, however, that the term has previously been used by palaeontologists with a slightly different meaning (Stanley, 1978). 

While the "chronospecies concept" calls for the establishment of temporal thresholds in general terms, it is the task of “(chrono-) species delimitation” to establish temporal threshold values (cutoffs) for the definition of species boundaries.

Since temporal threshold values are indispensable prerequisites for delimiting taxa of different ranks, they need to be included in future editions of the ICZN-Code. But how shall temporal threshold values for species delimitation be determined? This question will be addressed on page “Species delimitation”. 

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