Low-elevation regions harbour the majority of the world’s species diversity compared to high-elevation areas. This global gradient suggests that lowland species have had more time to diversify, or that net diversification rates have been higher in the lowlands. However, highlands seem to be cradles of diversity as they contain many young endemics, suggesting that their rates of speciation are exceptionally fast. Here we use a phylogenetic diversification model that accounts for the dispersal of species between different elevations to examine the evolutionary dynamics of the elevational diversity gradient in passerine birds, a group that has radiated globally to occupy almost all elevations and latitudes. We find strong support for a model in which passerines diversify at the same rate in the highlands and the lowlands but in which the per-capita rate of dispersal from high to low elevations is more than twice as fast as that in the reverse direction. This suggests that while there is no consistent trend in diversification across elevations, part of the diversity generated by highland regions migrates into the lowlands, thus setting up the observed gradient in passerine diversity. We find that this process drives tropical regions but for temperate areas, the analysis could be hampered by their lower richness. Despite their lower diversity, highland regions are disproportionally important for maintaining diversity in the adjacent lowlands.

与高海拔地区相比，低海拔地区拥有世界上大部分的物种多样性。这种全球梯度表明低地物种有更多时间进行多样化，或者低地的净多样化率更高。然而，高地似乎是多样性的摇篮，因为它们包含许多年轻的地方物种，这表明它们的物种形成速度异常快。在这里，我们使用系统发育多样化模型来解释物种在不同海拔之间的扩散，以检查雀形目鸟类海拔多样性梯度的进化动态，雀形目鸟类是一个辐射全球几乎占据所有海拔和纬度的群体。我们发现，雀形目鸟类在高地和低地以相同的速度多样化，但人均从高海拔向低海拔的扩散速度是反向扩散速度的两倍以上，我们发现该模型得到了有力支持。这表明，虽然高地之间的多样性没有一致的趋势，但高地地区产生的部分多样性迁移到低地，从而建立了观察到的雀形目多样性梯度。我们发现这个过程驱动热带地区，但对于温带地区，分析可能会因其较低的丰富度而受到阻碍。尽管多样性较低，但高地地区对于维持邻近低地的多样性非常重要。这表明，虽然高地之间的多样性没有一致的趋势，但高地地区产生的部分多样性迁移到低地，从而建立了观察到的雀形目多样性梯度。我们发现这个过程驱动热带地区，但对于温带地区，分析可能会因其较低的丰富度而受到阻碍。尽管多样性较低，但高地地区对于维持邻近低地的多样性非常重要。这表明，虽然高地之间的多样性没有一致的趋势，但高地地区产生的部分多样性迁移到低地，从而建立了观察到的雀形目多样性梯度。我们发现这个过程驱动热带地区，但对于温带地区，分析可能会因其较低的丰富度而受到阻碍。尽管多样性较低，但高地地区对于维持邻近低地的多样性非常重要。