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Open Access Highly Accessed Research article

Ring distributions leading to species formation: a global topographic analysis of geographic barriers associated with ring species

William B Monahan12*, Ricardo J Pereira134 and David B Wake14

Author Affiliations

1 Museum of Vertebrate Zoology, 3101 Valley Life Sciences Building, University of California, Berkeley, CA 94720, USA

2 National Park Service, Inventory and Monitoring Division, 1201 Oakridge Drive, Suite 150, Fort Collins, CO 80525, USA

3 CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Universidade do Porto, Campus Agrário de Vairão, 4485-661 Vairão, Portugal

4 Department of Integrative Biology, University of California, Berkeley, CA 94720, USA

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BMC Biology 2012, 10:20  doi:10.1186/1741-7007-10-20

Published: 12 March 2012

Additional files

Additional file 1:

Examples of predicted barriers and how they align with associated ecoregions or major oceanic features. Barriers are shown by red polygons, ecoregions by black outlines, and global elevations by the shaded topography. A: Zambezian flooded grasslands. B: Sichuan basin broadleaf evergreen forests. C: Kuh Rud and Eastern Iran montane woodlands. D: Eastern Guinean forests. E: Sea of Azov (SE Ukraine). F: southern portions of the Chilean matorral. G: multiple ecoregions in the Andes. H: multiple ecoregions on the Arabian Peninsula. I: South Equatorial Current. J: Falkland Current. K: Alaska Current. L: Bering Sea (North is oriented down).

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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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Additional file 3:

Principal component analysis. Principal component scores (PC1, PC2) for each summary statistic included in the topographic ring model.

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Additional file 4:

Barrier permeability as measured by fragmentation and shape. Left: barrier permeability (fragmentation) vs. size (area). Right: barrier permeability (shape as measured by the perimeter-to-area ratio) vs. size (area). Vertical lines identify barriers that are 50,000 km2. Gray points identify all barriers on the planet, as defined by the topographic model. Green points identify barriers where fragmentation < 1. Numbers correspond to barriers associated with known ring taxa: (1) Ensatina: Central Valley, California, USA; (2) Acacia: Drakensberg Massif, South Africa; (3) Larus: Makarov Basin, Arctic Ocean; (4) Phylloscopus: Tibetan Plateau, Central Asia; (5) Phylloscopus: Takla Maka and Gobi Deserts, Central Asia; (6) Larus: Amundsen and Nansen Basins, Arctic Ocean; (7) Larus: Canada Basin, Arctic Ocean. Smaller perimeter-to-area ratios describe barriers that are more circular and compact.

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Additional file 5:

Use of the topographic ring model to identify candidate taxa for ring diversification around a focal barrier in Costa Rica and Panama that is topographically similar to the reference barrier for the Central Valley (California, USA), which has promoted ring diversification in a salamander, Ensatina eschscholtzii. A: The focal barrier is a long-standing geographic feature known as the Cordillera de Talamanca. B: As a result of its particular topography, the mountainous barrier is surrounded at lower elevations by higher temperatures. C: In part due to these temperature gradients, the predicted barrier is considered a distinct ecoregion (Talamancan Montane Forests) that is surrounded by other distinct ecoregions, which form a ring distribution. D: These climatic and ecoregional conditions have shaped the distribution of many species, including the red-eyed tree frog, Agalychnis callidryas.

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Additional file 6:

Use of the topographic ring model to identify candidate taxa for ring diversification around a focal barrier in the Iberian Peninsula (southern Europe) that is topographically similar to the reference barrier for the Drakensberg Massif (South Africa), which has promoted ring diversification in a tree species, Acacia karroo. Extensive field-based studies in Iberia - particularly in reptiles and amphibians - have generated considerable distributional and phylogeographic data that can be used to evaluate whether the focal barrier has promoted continuous levels of differentiation typical of ring divergence. The focal barrier (top panel, map) is a long-standing geographic barrier for terrestrial organisms, serving as a steep ecotone between the main climatic regions of Iberia. As a result of its particular topography, a central arid and warmer plateau is surrounded by moister and colder habitat. These climatic conditions have shaped the distribution of many Atlantic species on the peninsula, including the fire salamanders Salmandra salamandra, and also Schreiber's green lizard Lacerta schreiberi, which forms a nearly complete ring distribution around the barrier (map). Extensive genetic data (in both mitochondrial and nuclear DNA) have been collected to reconstruct its phylogeographic history. In agreement with our model prediction, multi-locus data suggest that the focal barrier has strongly influenced non-adaptive divergence among currently contiguous populations of L. schreiberi, showing evidence of continuous levels of genetic differentiation around the barrier and no evidence of historical gene flow across it (bottom panel, phylogenetic network; thick branches are supported by > 0.95 posterior probability). Although the species in this example lacks terminal overlap, it illustrates how the topographic ring model may be used to properly identify and evaluate new instances of ring diversification.

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Additional file 7:

Use of the topographic ring model to identify candidate taxa for ring diversification around a focal barrier near the Baja California Peninsula (USA and Mexico) that is topographically similar to the reference barrier for the Drakensberg Massif (South Africa), which has promoted ring diversification in a tree species, Acacia karroo. The focal barrier (left panel, map) is a low-lying topographic depression located at the land-sea interface in the northern Sea of Cortez. As a result of its particular topography, the barrier has promoted diversification in a number of terrestrial taxa, including Hypsiglena nightsnakes and the rosy boa Lichanura trivirgata. In L. trivirgata, mitochondrial data have been collected to reconstruct its phylogeographic history. In agreement with our model prediction, these data suggest that the focal barrier has strongly influenced non-adaptive divergence among mostly contiguous subspecies of L. trivirgata, showing evidence of continuous levels of genetic differentiation along either side of the barrier (right panel, phylogenetic network; thick branches are supported by > 0.95 posterior probability). Closure of the ring distribution may occur in the northwest, between two deeply divergent lineages within the subspecies roseofusca (symbolized by circles and hexagons).

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Additional file 8:

Use of the topographic ring model to identify candidate taxa for ring diversification around a focal barrier on the island of New Guinea that is topographically similar to the reference barrier for the Central Valley (California, USA), which has promoted ring diversification in a salamander, Ensatina eschscholtzii. The focal barrier (upper right panel, map) is a mountain forest ecoregion that is surrounded at lower elevations by warmer and generally drier ecoregions and basins. This distribution of contrasting bioclimates is hypothesized to have promoted diversification in a number of bird taxa, including Pitohui, Tanysiptera kingfishers, Aegotheles owlet-nightjars, and the little shrike-thrush Colluricincla megarhyncha. All of these taxa are monophyletic and have diversified around the barrier, reaching different stages of divergence. This diversification is especially well illustrated by C. megarhyncha, where mitochondrial data have been collected to reconstruct its phylogeographic history. In agreement with our model prediction, these data suggest that the focal barrier has strongly influenced non-adaptive divergence among mostly contiguous subspecies of C. megarhyncha, showing evidence of continuous levels of genetic differentiation along either side of the barrier (left panel, phylogenetic network; numbers report numbers of site or base pair substitutions between haplotypes). Additionally, plots of genetic vs. geographic distance (lower right panel, plot) reveal significant isolation by distance around the barrier, but not across it, suggesting that this is an important barrier to colonization and gene flow. Although it is unclear whether there is terminal overlap at the southern end of the ring distribution, this example illustrates how the topographic ring model may be used to properly identify and evaluate new instances of ring diversification.

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Additional file 9:

Use of the topographic ring model to identify candidate taxa for ring diversification around a focal barrier in Madagascar that is topographically "in between" (Figure 3) reference barriers for the Drakensberg Massif (South Africa), which has promoted ring diversification in a tree species, Acacia karroo, and the Tibetan Plateau (Central Asia), which has promoted ring diversification in a bird species, Phylloscopus trochiloides. The focal barrier (right panel, map) is a mountainous subhumid bioclimatic zone surrounded at lower elevations by humid (east) and subarid/dry (west) zones. This distribution of contrasting bioclimates is hypothesized to have promoted diversification in amphibians, reptiles, and lemurs, some of which form either complete or nearly complete ring distributions around the barrier. In Propithecus lemurs, mitochondrial data have been collected to reconstruct its phylogeographic history. In agreement with our model prediction, these data suggest that the focal barrier has strongly influenced non-adaptive divergence among mostly contiguous species of Propithecus, showing evidence of continuous levels of genetic differentiation (from north to south) along either side of the barrier (left panel, phylogenetic tree; thick branches are supported by > 0.95 posterior probability). Although there appears to be no overlap of terminal taxa in the south, this example illustrates how the topographic ring model may be used to properly identify and evaluate new instances of ring diversification.

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