10 × (times) average intraspecific difference
||Hebert PD, Stoeckle MY, Zemlak TS, Francis CM. Identification of Birds through DNA Barcodes. PLoS Biol. 2004 Oct2(10):e312. DOI: 10.1371/journal.pbio.0020312 abstract & p.1661 right column bottom paragraphPubMed ID15455034
||Abstract: "Recent work suggests that sequence diversity in a 648-bp region of the mitochondrial gene, cytochrome c oxidase I (COI), might serve as a DNA barcode for the identification of animal species. This study tested the effectiveness of a COI barcode in discriminating bird species, one of the largest and best-studied vertebrate groups. [Investigators] determined COI barcodes for 260 species of North American birds and found that distinguishing species was generally straightforward."
||Abstract: "The finding of large COI sequence differences between, as compared to small differences within, species confirms the effectiveness of COI barcodes for the identification of bird species. This result plus those from other groups of animals imply that a standard screening threshold of sequence difference (10x average intraspecific difference) could speed the discovery of new animal species." P.1661 right column bottom paragraph: "What threshold might be appropriate for flagging genetically divergent specimens as provisional species? This threshold should certainly be high enough to separate only specimens that very likely belong to different species. Because patterns of intraspecific and interspecific variation in COI appear similar in various animal groups (Grant and Bowen 1998 [sardines] Hebert et al. 2003a [moths] Hogg and Hebert 2004 [springtails]), [investigators] propose a standard sequence threshold: 10× the mean intraspecific variation for the group under study. If applied to the birds examined in this study (0.27% average intraspecific variation 2.7% threshold), a 10× threshold would recognize over 90% of the 260 known species, as well as the four probable new species. As this result demonstrates, a threshold approach will overlook species with short evolutionary histories and those exposed to recent hybridization, but it will be a useful screening tool, especially for groups that have not received intensive taxonomic analysis."