Additional file 2.

Additional ring-distributed taxa surrounding reference barriers. A: The bird species complex Alauda (sp. arvensis and gulgula): Central Asia. B: The bird species Parus major: Central Asia. C: The bird species complex Charadrius (sp. hiaticula and semipalmatus): Arctic Ocean. Barriers are shown by the red polygons, species' distributions by the black points and gray polygons (Charadrius), and global elevations by the shaded topography. Numbers correspond to individual barriers identified in the PCA.

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Monahan et al. BMC Biology 2012 10:20   doi:10.1186/1741-7007-10-20