I want to share a few quotes about DNA barcoding. Some of these are from critics of barcoding (Roe and Sperling 2007) and some are from proponents of DNA barcoding (Hebert and Gregory 2005). For fun, see if you can guess which is which, and then see if you can guess why critics keep saying the same thing despite what barcoding actually involves.
This shortfall suggests that mtDNA is not as universally successful as suggested by DNA-barcoding advocates. Additional markers are clearly essential to clarify species boundaries, providing additional support for an integrative, multilocus approach to delimiting species.
There is an important distinction between “describing” and “delimiting” species, but a conflation of the two has created uneasiness about the use of DNA barcodes as the foundation of future taxonomic descriptions. We emphasize that DNA barcoding seeks merely to aid in delimiting species – to highlight genetically distinct groups exhibiting levels of sequence divergence suggestive of species status. By contrast, DNA barcodes – by themselves – are never sufficient to describe new species. At some stage, clearly divergent DNA barcodes, in combination with other information, will be used as the basis for providing a new Linnaean name and, as with any taxonomic hypothesis, this would be subject to ongoing re-evaluation. For example, in a recent survey of North American birds, the threshold for delineating probable new species was arbitrarily set at 10x the average within-species variation of the entire barcode dataset. This led to the revelation of four presumptive new species, but decisions regarding the formal recognition of these taxa are left, appropriately, to the ornithological community (notably, existing morphological and behavioural information supports these new hypotheses).
Rather than concentrating on a single character type (e.g. morphological or DNA sequence variation), a wide range of characters, including molecular data, morphology, behaviour, and geography, are integral to assessing species boundaries and delimiting species.
DNA barcoding using a single gene region does not assure complete taxonomic resolution, but it does promise proximity. Based on past results for varied animal groups, DNA barcoding will deliver species-level resolution in 95-97% of cases. When it fails, it will narrow the options to a small number of congeneric taxa (which, in many cases, could be resolved fully with additional genetic or other data).
Additional sources of data, such as morphology, host preference, geographic range and pheromone can be used when elucidating species boundaries, as this approach will improve detection of incongruence between markers, allowing insight into the process of speciation.
The synergy between DNA barcoding and studies of morphological/ecological diversity is further illustrated by the case of the skipper butterfly, Astraptes fulgerator, in which a combined morphological, natural history and barcoding approach revealed a complex of 10 species in one small area of
. Importantly, several of these species showed a relatively small barcode divergence, but a coupling of this information with records on larval host plants and morphology illuminated the full diversity of the complex. Costa Rica
Hebert, P.D.N. and T.R. Gregory. 2005. The promise of DNA barcoding for taxonomy. Systematic Biology 54: 582-859.
Roe, A.D. and F.A.H. Sperling. 2007. Population structure and species boundary delimitation of cryptic Dioryctria moths: an integrative approach. Molecular Ecology 16: 3617-3633.