Closely related species with parapatric elevational ranges are ubiquitous in tropical mountains worldwide. The gradient speciation hypothesis proposes that these series are the result of in situ ecological speciation driven by divergent selection across elevation. Direct tests of this scenario have been hampered by the difficulty inferring the geographic arrangement of populations at the time of divergence. In cichlids, sticklebacks and Timema stick insects, support for ecological speciation driven by other selective pressures has come from demonstrating parallel speciation, where divergence proceeds independently across replicated environmental gradients. Here, we take advantage of the unique geography of the island of New Guinea to test for parallel gradient speciation in replicated populations of Syma kingfishers that show extremely subtle differentiation across elevation and between historically isolated mountain ranges. We find that currently described high‐elevation and low‐elevation species have reciprocally monophyletic gene trees and form nuclear DNA clusters, rejecting this hypothesis. However, demographic modelling suggests selection has likely maintained species boundaries in the face of gene flow following secondary contact. We compile evidence from the published literature to show that although in situ gradient speciation in labile organisms such as birds appears rare, divergent selection and post‐speciation gene flow may be an underappreciated force in the origin of elevational series and tropical beta diversity along mountain slopes.

中文翻译：

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跨越新几内亚翠鸟海拔梯度的物种形成和基因流

具有近缘海拔范围的密切相关物种在世界各地的热带山脉中无处不在。梯度物种形成假说认为，这些系列是由跨海拔不同选择驱动的原位生态物种形成的结果。由于难以推断分歧时人口的地理分布，因此对这种情况的直接测试受到了阻碍。在慈鲷、棘鱼和 Timema 竹节虫中，对由其他选择压力驱动的生态物种形成的支持来自展示平行物种形成，其中分歧在复制的环境梯度中独立进行。这里，我们利用新几内亚岛独特的地理优势来测试 Syma 翠鸟复制种群的平行梯度物种形成，这些种群在海拔高度和历史上孤立的山脉之间显示出极其细微的差异。我们发现目前描述的高海拔和低海拔物种具有相互单系的基因树并形成核 DNA 簇，否定了这一假设。然而，人口模型表明，在二次接触后的基因流动面前，选择可能保持了物种边界。我们从已发表的文献中收集证据表明，虽然鸟类等不稳定生物的原位梯度物种形成似乎很少见，