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

BACKGROUND: In the mid 20th Century, Ernst Mayr and Theodosius Dobzhansky championed the significance of circular overlaps or ring species as the perfect demonstration of speciation, yet in over 50 years since only a handful of such taxa are known. We developed a topographic model to evaluate whether the geographic barriers that favor processes leading to ring species are common or rare, and to predict where other candidate ring barriers might be found. RESULTS: Of the 952,147 geographic barriers identified on the planet, only about 1% are topographically similar to barriers associated with known ring taxa, with most of the likely candidates occurring in under-studied parts of the world (e.g., marine environments, tropical latitudes). Predicted barriers separate into two distinct categories: (i) single cohesive barriers (<50,000 km2), associated with taxa that differentiate at smaller spatial scales (salamander: Ensatina eschscholtzii; tree: Acacia karroo); and (ii) composite barrier...