Biodiversity Assessment, DNA Barcoding, and the Minority Majority

The majority of species on Earth are in "under-studied" groups, and indeed probably the majority of species remain undiscovered and undescribed. Species are natural units of evolution, and they are formed from branching phylogenetic processes that have a mathematical structure. So it follows that we should be able to develop a set of general principles that describe global patterns of species groups, like genera. Understanding such patterns would lend considerable power to the approach of "taxonomic surrogacy." In environmental assessments, ecology, and paleontology, it is common to substitute genus-level or family-level identification where definitive species identification is impractical. Clarity and confidence in fundamental patterns, based on a robust null model for species and genus level diversity, can accelerate species discovery: there are more species in the tropics, species-poor genera are very common, large genera are rare. Much hope has been placed in DNA barcoding as an effective tool to increase the pace of species discovery, but it is abundantly clear that certain mitochondrial DNA (mtDNA) markers are more or less variable in different clades and universal threshold values are impractical to delimit species. This study further examines the patterns of divergence in one common mtDNA barcode fragment, cytochrome c oxidase subunit 1at the genus level. We compared pairwise divergence in this fragment between two animal clades that have similar species richness but different evolutionary histories: birds and bivalves. We analyzed quality controlled alignments of over 39,000 published sequences in 1223 genera. Median pairwise differences at the genus level are positively correlated with the species richness of a genus, and this is not dependent of the number of sequences sampled. Unsurprisingly, sequence divergence in vertebrates was far more constrained than in evolutionarily more ancient non-vertebrate clades. Differences among the groups examined highlight the need for DNA barcode approaches to be considered in the context of specific biological groups. Vertebrates are better studied, but not necessarily representative of the majority of biodiversity. A technique that provides powerful insights for vertebrate species may be ineffective for the majority of organisms.