A presumed genotypic threshold of 4 was used for delineating genetic distance in birds, based on the computation of twice the mitochondrial DNA rate of 2% sequence divergence per million years, as noted in Price (2007, Speciation in birds).
There are no thresholds. In some cases, valid species may have no differentiation at all in their barcode or virtually any other gene. In others, populations of conspecific taxa may have easily up to 7% divergence in their barcode, that not meaning that they should be considered distinct (ancient hybirdization, incomplete lineage sorting, past isolation and subsequent remix of populatios and so on). Criteria for species status assessment are as Madhava's reply.
Forgot to mention that it is important to remember the causation-correlation issue: which is first: speciation or differentiation? Both are processes that may be dependent or independent of each other. And that is case-sensitive, so one cannot generalize, unfortunately, anything from even closely related other taxa of any 'thresholds'.
Thank you very much for your invaluable response, Madhava and Lauri. Much appreciated indeed. I believe 4% might be too low for Cytb as well, but a definitive threshold may not be fully realised given there are too many issues to consider. But for purposes of comparison between genotype and phenotype, I was hoping to find a doable standard to follow, and compare with the criteria presented by Tobias et al. 2010 in Ibis, which uses a threshold of 7.0 to delimit species.
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