Patterns of speciation are similar across mountainous and lowland regions for a Neotropical plant radiation (Costaceae: Costus)

High species richness and endemism in tropical mountains are recognized as major contributors to the latitudinal diversity gradient. The processes underlying mountain speciation, however, are largely untested. The prevalence of steep ecogeographic gradients and the geographic isolation of populations by topographic features are predicted to promote speciation in mountains. We evaluate these processes in a species-rich Neotropical genus of understory herbs that range from the lowlands to montane forests and have higher species richness in topographically complex regions. We ask whether climatic niche divergence, geographic isolation, and pollination shifts differ between mountain-influenced and lowland Amazonian sister pairs inferred from a 756-gene phylogeny. Neotropical Costus ancestors diverged in Central America during a period of mountain formation in the last 3 My with later colonization of Amazonia. Although climatic divergence, geographic isolation, and pollination shifts are prevalent in general, these factors don't differ between mountain-influenced and Amazonian sister pairs. Despite higher climatic niche and species diversity in the mountains, speciation modes in Costus appear similar across regions. Thus, greater species richness in tropical mountains may reflect differences in colonization history, diversification rates, or the prevalence of rapidly evolving plant life forms, rather than differences in speciation mode.

热带山地极高的物种丰富度与特有性，被认为是纬度多样性格局形成的核心驱动因素之一。然而，山地物种形成背后的核心过程迄今尚未得到充分验证。学界普遍推测，陡峭的生态地理梯度与地形特征造成的种群地理隔离，能够推动山地物种形成过程。本研究以新热带区一个物种丰富的林下草本属为研究对象，该属植物的分布范围涵盖低地至山地森林，且在地形复杂区域拥有更高的物种丰富度，以此评估上述山地物种形成相关过程。本研究基于756个基因构建的系统发育树，筛选出受山地影响的类群与亚马孙低地类群的姊妹类群对，旨在探究气候生态位分化、地理隔离与传粉模式转变这三类因素在两类姊妹类群间是否存在差异。新热带区闭鞘姜属（Costus）的祖先类群于距今300万年的山地形成时期在中美洲发生分化，后续才扩张至亚马孙地区。尽管总体而言，气候生态位分化、地理隔离与传粉模式转变这三类过程普遍存在，但上述因素在受山地影响的类群与亚马孙类群的姊妹对之间并无显著差异。尽管山地的气候生态位与物种多样性更高，但闭鞘姜属的物种形成模式在不同区域间并无明显差异。因此，热带山地更高的物种丰富度，可能反映的是殖民历史、多样化速率或快速演化植物生活型的占比差异，而非物种形成模式本身的区别。