Data from: 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 barriers – formed by groups of barriers (each 184,000 to 1.7 million km2) in close geographic proximity (totaling 1.9 to 2.3 million km2) – associated with taxa that differentiate at larger spatial scales (birds: Phylloscopus trochiloides and Larus [sp. argentatus and fuscus]). When evaluated globally, we find a large number of cohesive barriers that are topographically similar to those associated with known ring taxa. Yet, compared to cohesive barriers, an order of magnitude fewer composite barriers are similar to those that favor ring divergence in species with higher dispersal. CONCLUSIONS: While these findings confirm that the topographic conditions that favor evolutionary processes leading to ring speciation are in fact rare, they also suggest that many understudied natural systems could provide valuable demonstrations of continuous divergence towards the formation of new species. Distinct advantages of the model are that it (i) requires no a priori information on the relative importance of features that define barriers, (ii) can be replicated using any kind of continuously distributed environmental variable, and (iii) generates spatially explicit hypotheses of geographic species formation. The methods developed here – combined with study of the geographical ecology and genetics of taxa in their environments – should enable recognition of ring species phenomena throughout the world.

研究背景：20世纪中期，恩斯特·迈尔（Ernst Mayr）与西奥多西尼斯·杜布赞斯基（Theodosius Dobzhansky）将环形重叠或环种（ring species）推崇为物种形成的绝佳实证案例，但此后五十余年间，已知的此类类群仅寥寥数种。我们构建了地形模型（topographic model），以评估利于环种形成过程的地理屏障（geographic barriers）究竟常见抑或罕见，并预测其他潜在环种屏障的分布区域。
研究结果：全球范围内已识别的952147处地理屏障中，仅约1%的屏障在地形特征上与已知环物种类群相关的屏障相似，且多数潜在候选屏障分布于研究不足的区域（如海洋环境、热带纬度地带）。预测得到的屏障可划分为两类截然不同的类别：（i）单一连片屏障（面积小于50000平方千米），对应空间分化尺度较小的类群（如蝾螈：Ensatina eschscholtzii；金合欢树：Acacia karroo）；（ii）复合屏障——由地理上紧密相邻的多组屏障构成（单组屏障面积介于184000至170万平方千米，总面积达190万至230万平方千米），对应空间分化尺度较大的类群（如鸟类：Phylloscopus trochiloides 以及 Larus 属[argentatus与fuscus种]）。经全球范围评估，我们发现与已知环物种类群地形特征相似的单一连片屏障数量可观，但相较单一连片屏障，与高扩散能力物种的环化分化相关的复合屏障数量仅为其十分之一左右。
研究结论：本研究结果证实，利于环化物种形成进化过程的地形条件实则罕见，但同时也表明，诸多研究不足的自然系统有望为新物种形成过程中的连续分化提供极具价值的实证案例。该模型的显著优势在于：（i）无需预先获取界定屏障的特征的先验信息；（ii）可借助任意类型的连续分布环境变量进行复现；（iii）能够生成地理物种形成的空间显性假说。本文所开发的方法结合类群地理生态学与环境中类群遗传学研究，将助力全球范围内环物种现象的识别。