Theoretical species concept and operational species criteria

Guided by the idea that DNA should be informative at all taxonomic levels, I conclude that the delimitation of species and subspecies should also be based on differences in DNA sequences (as discussed in Jarvis, 2016 and recommended for allopatric species by Price, 2008 and Hudson & Price, 2014). This pragmatic approach is based on the widely accepted assumption that the speciation process represents a continuum that can only be arbitrarily divided. It ultimately follows that species cannot be detected but have to be defined. Originally established in palaeontology (Stanley, 1978), I will expand the meaning of the term “chronospecies” by applying it to the DNA-based species concept to be outlined below. 

First, differences in the DNA signature of independent lineages need to be quantified. Next, the observed differences need be translated into divergence times. Finally, the putative divergence times are used to delimit species and subspecies according to the putative times of divergence. Species status might be attributed to lineages that are at least two million years old (following Price, 2008; and Hudson & Price, 2014), and subspecies status to lineages that are at least one million years old ( new suggestion). 

Age should be the sole operating criterion in defining species and subspecies. At present, however, avian taxonomists integrate everything - except time. Other traits (morphology, acoustics, behaviour, habitat preference, distribution, and possibly others) are still required to investigate the underlying evolutionary processes of speciation. 

Relying exclusively on age substantially reduces the level of subjectivity in species-delimitation decisions. One delimiting ages for species and subspecies are agreed upon, species-level taxonomies could be generated automatically by computer programs. 



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