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）与西奥多修斯·杜布赞斯基（Theodosius Dobzhansky）大力倡导环物种（ring species）及环状重叠现象作为物种形成的完美实证案例，但此后五十余年间，学界仅发现寥寥数例此类类群。本研究构建了地形模型（topographic model），用以评估有利于环物种形成过程的地理屏障（geographic barriers）究竟是常见还是稀有，并预测其他潜在的环物种屏障分布区域。

研究结果：全球范围内已识别的952147个地理屏障中，仅约1%的地形与已知环物种类群相关的屏障相似，其中多数潜在候选屏障分布于全球研究不足的区域（例如海洋环境、热带纬度地带）。预测得到的屏障可分为两类截然不同的类别：（一）单一黏合屏障（cohesive barriers）（<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*Acacia karroo*；（二）复合屏障（composite barriers）：由地理上邻近的多个屏障组构成（单个屏障面积184000至170万平方千米，总面积190万至230万平方千米），对应空间分化尺度较大的类群，如鸟类冕柳莺*Phylloscopus trochiloides*以及鸥属*Larus*（*argentatus*与*fuscus*种）。全球尺度评估结果显示，存在大量与已知环物种类群相关屏障地形相似的单一黏合屏障。然而与单一黏合屏障相比，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。

结论：本研究结果证实，有利于环物种形成（ring speciation）演化过程的地形条件确实较为罕见，但同时也表明，全球诸多研究不足的自然生态系统，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：（一）无需预先知晓定义地理屏障的各项特征的相对重要性；（二）可基于任意一类连续分布的环境变量进行复现；（三）能够生成地理物种形成过程的空间显性假说。本研究提出的方法结合类群生存环境中的地理生态学与遗传学研究，有望在全球范围内识别环物种现象。