Growing barcode libraries confirm distinct clustering of mtDNA sequences. In early surveys of mtDNA differences, it seemed possible that as larger numbers of individuals were sampled, there would invevitably be many overlaps between closely-related species. The accumulating barcode data show this supposition is incorrect. Instead, further sampling reinforces the observation that most animal species correspond to distinct mtDNA sequence clusters, as for example, in the tree of Canada and Cackling Goose mtDNA sequences below (Figure A generated with public data files and software on Barcode of Life Database site https://www.barcodinglife.org/). Where large differences within species are found, they generally reflect the distinct sequence clusters of geographically restricted populations which have other identifiable biological differences, as in tree of Winter Wren mtDNA sequences below. Such clusters are probably best regarded as separate species (Figure B adapted from Drovetski et al 2004 Proc R Soc Lond B 271:545; number individuals sequenced shown in parentheses).
An emerging corollary is that most named subspecies do not represent evolutionary significant units. Large-scale surveys are revealing many genetically distinct clusters within named species, but these clusters generally do NOT correspond to described subspecies. For example, none of the 39 subspecies of Winter wren correspond to the geographic clades reported by Drovetski et al. and grouping by subspecies did not account for any variance. Robert Zink reports that “97% of continentally distributed avian subspecies lack the population genetic structure indicative of a distinct evolutionary unit” (Zink 2004 Proc R Soc Lond B 271:564). Regarding mtDNA sequence clusters, Zink states “it is these unnamed units and not named subspecies that should play a major role in guiding conservation efforts and in identifying biological diversity” Large-scale DNA barcode surveys of multicellular animals and plants can provide a foundation for intelligent conservation efforts.