Figure 1: Figure 1. Comparison of Nucleotide Sequence Differences in COI among 260 Species of North American birds Adapted from: Hebert PDN, Stoeckle MY, Zemlak TS, Francis CM (2004) Identification of Birds through DNA Barcodes. PLoS Biol 2(10): e312.

Comparison of DNA differences among North American Birds

The original research paper states: "Pairwise comparisons between 437 COI sequences are separated into three categories: differences between individuals in the same species, differences between individuals in the same genus (not including intraspecific differences), and differences between individuals in the same family (not including intraspecific or intrageneric differences)."

What does the figure mean? Researchers used the DNA sequence from the COI gene in birds to figure out if the COI gene is useful as a DNA barcode-- in other words, a unique identifier for bird species.

For the "Within Species" analysis (top panel), researchers compared one bird's 640-base pair COI sequence with another sequence from a bird of the same species. They recorded the number of differences between the two sequences (identical sequences would have zero differences, whereas completely different sequences would have 640 differences).

They repeated this for all of their bird sequences. These comparisons are called "pairwise" because the data they recorded was based on each pair of sequences compared. In the graph, the x-axis shows the number of differences between sequences, and the y-axis shows the percentage of comparisons.

For the "Within Genus" analysis (middle panel), researchers compared one bird COI sequence with all the others of the same genus. Again, they recorded the number of differences between the sequences, and then repeated this analysis for all the sequences. Since members of the same genus might also be members of the same species, they didn't count those comparisons. The same method was used for "Within Family" comparisons.

Identification of Birds through DNA Barcodes

Paul D. N. Hebert1* , Mark Y. Stoeckle2 , Tyler S. Zemlak1 , Charles M. Francis3

1 Department of Zoology, University of Guelph, Guelph, Ontario, Canada, 2 Program for the Human Environment, Rockefeller University, New York, New York, United States of America, 3 National Wildlife Research Centre, Canadian Wildlife Service, Ottawa, Ontario, Canada

DOI: 10.1371/journal.pbio.0020312 Copyright: © 2004 Hebert et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Citation: Hebert PDN, Stoeckle MY, Zemlak TS, Francis CM (2004) Identification of Birds through DNA Barcodes. PLoS Biol 2(10): e312.