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 million years with later colonization of Amazonia. Although climatic divergence, geographic isolation, and pollination shifts are prevalent in general, these factors do not 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.

Author Contributions: Study was designed by OMV, DLG and KMK. KMK curated the plant collection used for genomic sampling. OMV, DLG and KMK collected the data. Phylogenetic analyses were conducted by OMV. Species richness and topographic complexity analyses were conducted by BG; all other geographic and climatic analyses were conducted by DLG. Juarez (UCSC) for help with field collecting, and Edgardo Ortiz for assistance with the analysis of the transcriptomic data. All field research was conducted with appropriate collection permits in Costa Rica (M AbstractHigh 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 formation of steep ecogeographic gradients and the geographic isolation of populations by geological features are predicted to promote speciation in mountains relative to lowland regions. We evaluate these processes in a species-rich Neotropical genus of understory herbs, Costus, that range from the lowlands to montane forests. We test for an association between topography and species richness, infer a biogeographical history, and ask whether the contributions of niche divergence, geographical isolation, and pollination shifts differ between mountain-influenced and Amazonian sister pairs inferred from a 756-gene phylogeny. We find that species richness is highest in topographically complex regions and infer that early Costus ancestors diverged in Central America during a period of mountain formation in the last 3 My.We find evidence for climatic divergence, geographical isolation, and pollination shifts in general, but do not find significant differences in these factors between mountain-influenced and Amazonian sister pairs. We conclude that despite higher indices of climatic niche and species diversity in the mountains, speciation patterns in Costus appear similar across tropical lowland and mountainous regions.

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