TRENDS in Genetics
DNA barcoding: how it complements taxonomy, molecular phylogenetics and population genetics
Mehrdad Hajibabaei1, Gregory A.C. Singer2, Paul D.N. Hebert1 and Donal A. Hickey3
Biodiversity Institute of Ontario, Department of Integrative Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada Human Cancer Genetics Program, The OhioState University, Columbus, OH 43210, USA 3 Department of Biology, Concordia University, 7141 Sherbrooke Street, Montreal, Quebec H4B 1R6, Canada
DNA barcoding aims to provide an efﬁcient method for species-level identiﬁcations and, as such, will contribute powerfully to taxonomic and biodiversity research. As the number of DNA barcode sequences accumulates, however, these data will also provide aunique ‘horizontal’ genomics perspective with broad implications. For example, here we compare the goals and methods of DNA barcoding with those of molecular phylogenetics and population genetics, and suggest that DNA barcoding can complement current research in these areas by providing background information that will be helpful in the selection of taxa for further analyses. Introduction Becauseof advances in sequencing and computational technologies, DNA sequences have become the major source of new information for advancing our understanding of evolutionary and genetic relationships. The footprints of comparative sequence analysis are now apparent in almost all areas of the biological sciences, from development to epidemiology . However, two branches of biology have developed thetools and applications employed to assess biological relationships with DNA sequences: molecular phylogenetics, and population genetics. These disciplines focus on different levels of organization. Studies in molecular phylogenetics typically deal with evolutionary relationships among deeper clades, whereas those in population genetics target variation within and among populations of a singlespecies. By comparison, DNA barcoding occupies a middle ground as it seeks comprehensive coverage for species, but focuses on their delineation rather than their relationships (Figure 1). DNA barcoding is based on the premise that a short standardized sequence can distinguish individuals of a species because genetic variation between species exceeds that within species . Pilot projects have nowestablished the effectiveness of this approach in several large groups of animals, such as birds , ﬁsh , cowries , spiders , and several arrays of Lepidoptera [7–9]. In addition, DNA barcoding systems are now being established for other groups of organisms, including plants , macroalgae , fungi , protists  and bacteria . DNA
Corresponding author: Hajibabaei, M.(email@example.com). Available online 20 February 2007.
barcoding datasets are essentially composed of short DNA sequences from several individuals of a large number of species (typically ﬁve to ten individuals per species, but these numbers will increase in the future) (Figure 1). Here, we discuss the role of DNA barcodes in advancing the taxonomic enterprise and itspotential to provide a contextual framework for both building phylogenies and for population genetics. In particular, we argue that barcode results can be of high value in aiding the selection of species for more detailed analysis, and demonstrate that DNA barcoding can broaden our understanding of both phylogenetic signal and population-level variation. The DNA barcoding workﬂow Species identiﬁcationthrough barcoding is usually achieved by the retrieval of a short DNA sequence – the ‘barcode’ – from a standard part of the genome (i.e. a speciﬁc gene region) from the specimen under investigation. The barcode sequence from each unknown specimen is then compared with a library of reference barcode sequences derived from individuals of known identity (Figure 2). A specimen is identiﬁed if its...