Data from: Why are tropical mountain passes ‘low’ for some species? genetic and stable-isotope tests for differentiation, migration, and expansion in elevational generalist songbirds

1.Most tropical bird species have narrow elevational ranges, likely reflecting climatic specialization. This is consistent with Janzen's Rule, the tendency for mountain passes to be effectively ‘higher’ in the tropics. Hence, those few tropical species that occur across broad elevational gradients (elevational generalists) represent a contradiction to Janzen's Rule.

2.Here we aim to address the following questions. Are elevational generalists being sundered by diversifying selection along the gradient? Does elevational movement cause these species to resist diversification or specialization? Have they recently expanded, suggesting that elevational generalism is short-lived in geological time?

3.To answer these questions, we tested for differentiation, movement, and expansion in four elevational generalist songbird species on the Andean west slope. We used morphology and mtDNA to test for genetic differentiation between high- and low-elevation populations. To test for elevational movements, we measured hydrogen isotope (δ2H) values of metabolically inert feathers and metabolically active liver.

4.Morphology differed for House Wren (Troglodytes aedon) and Hooded Siskin (Spinus magellanicus), but not for Cinereous Conebill (Conirostrum cinereum) and Rufous-collared Sparrow (Zonotrichia capensis), respectively. mtDNA was structured by elevation only in Z. capensis. δ2H data indicated elevational movements by two tree- and shrub-foraging species with moderate-to-high vagility (C. cinereum and S. magellanicus), and sedentary behavior by two terrestrial-foraging species with low-to-moderate vagility (T. aedon and Z. capensis). In S. magellanicus, elevational movements and lack of mtDNA structure contrast with striking morphological divergence, suggesting strong diversifying selection on body proportions across the ~50 km gradient. All species except C. cinereum exhibited mtDNA-haplotype variation consistent with recent population expansion across the elevational gradient, potentially concurrent with Holocene anthropogenic habitat conversion for agriculture.

5.In different ways, each species defies the tendency for tropical birds to have long-term stable distributions and sedentary habits. We conclude that tropical elevational generalism is rare due to evolutionary instability.

1. 绝大多数热带鸟类仅拥有狭窄的海拔分布范围，这大概率反映了其对特定气候的适应性特化。这一现象与詹森法则（Janzen's Rule）相符——即热带地区的山地隘口实际海拔“高度”更高（意即跨海拔扩散难度更大）。因此，极少数能跨越宽泛海拔梯度的热带物种（即海拔泛化种（elevational generalists）），便与詹森法则形成了矛盾。
2. 本研究旨在解答以下三个问题：其一，沿海拔梯度产生的分化选择是否正在割裂这些海拔泛化种？其二，海拔移动是否会阻碍这些物种发生分化或适应性特化？其三，这些物种是否在近期发生了种群扩张，从而表明海拔泛化策略在地质时间尺度上仅为短暂存在？
3. 为解答上述问题，我们针对安第斯山脉西坡的4种海拔泛化鸣禽，开展了分化、移动与种群扩张相关检测。我们通过形态学数据与线粒体DNA（mtDNA），检测了高海拔与低海拔种群间的遗传分化。为检测海拔移动情况，我们测定了代谢惰性羽毛与代谢活跃肝脏中的氢同位素（δ²H）值。
4. 形态学差异仅见于家鹪鹩（Troglodytes aedon）与黑顶黄雀（Spinus magellanicus），而灰锥嘴雀（Conirostrum cinereum）和棕顶雀鹀（Zonotrichia capensis）未表现出显著形态差异。仅棕顶雀鹀（Z. capensis）的线粒体DNA（mtDNA）结构受海拔影响。氢同位素（δ²H）数据显示，2种以树木和灌丛为觅食生境、具有中等到较高移动能力的物种（灰锥嘴雀C. cinereum与黑顶黄雀S. magellanicus）存在海拔移动行为，而2种以陆地为觅食生境、移动能力低至中等的物种（家鹪鹩T. aedon与棕顶雀鹀Z. capensis）则表现出定居习性。在黑顶黄雀S. magellanicus中，海拔移动行为与线粒体DNA结构缺乏分化的特征，与其显著的形态分化形成鲜明对比，这表明在约50公里的海拔梯度上，其身体比例受到了强烈的分化选择。除灰锥嘴雀C. cinereum外，其余所有物种均表现出线粒体DNA单倍型变异，与跨海拔梯度的近期种群扩张相符，这一扩张可能与全新世人类活动导致的农业生境改造同期发生。
5. 各物种均以不同方式，打破了热带鸟类长期拥有稳定分布范围与定居习性的普遍趋势。我们由此得出结论：热带地区的海拔泛化策略之所以罕见，根源在于其进化上的不稳定性。