Reference barriers and reference taxa. A: The salamander species Ensatina eschscholtzii: Central Valley, California, USA. B: The bird species Phylloscopus trochiloides: Central Asia. C: The tree species Acacia karroo: Drakensberg Massif, South Africa. D: The bird species complex Larus: Arctic Ocean. Barriers are shown by the red polygons, species' distributions by the black points, and global elevations by the shaded topography. Numbers correspond to individual barriers identified in the PCA.