Testing evolutionary hypotheses for DNA barcoding failure in willows.

The goal of DNA barcoding is to enable the rapid identification of taxa from short diagnostic DNA sequence profiles. But how feasible is this objective when many evolutionary processes, such as hybridization and selective sweeps, cause alleles to be shared among related taxa? In this issue of Molecular Ecology, Percy et al. (2014) test the full suite of seven candidate plant barcoding loci in a broad geographic sample of willow species. They show exceptional plastid haplotype sharing between species across continents, with most taxa not possessing a unique barcode sequence. Using population genetic and molecular dating analyses, they implicate hybridization and selective sweeps, but not incomplete lineage sorting, as the historical processes causing widespread haplotype sharing among willow taxa. This study represents an exceptional case of how poorly barcoding can perform, and highlights methodological issues using universal organellar regions for species identification.