Exploring Andean foothill connectivity: insights from population divergence in the white-tipped sicklebill, Eutoxeres aquila

The Northern Andes, acting both as barrier and corridor, exert strong influence in Neotropical biodiversity. Still, the diversification of foothill taxa inhabiting ecotonal zones between lowland rainforest and montane environments in this region remains poorly understood. We investigated the evolutionary history of Eutoxeres aquila, a hermit hummingbird distributed across Central America, the Chocó, and both Andean slopes. Using mitochondrial DNA (ND2 and cytb), morphometrics, resistance surface analyses, and ecological niche models, we examined the potential roles of topography, connectivity, and climatic oscillations in shaping lineage divergence. Phylogenetic analyses of 59 individuals of Eutoxeres aquila revealed two major clades: 1) an eastern clade (eastern Andean slope–upper Magdalena valley + mid Magdalena valley–Tamá), and 2) a western clade (western slope–N Central cordillera + Panama). Divergence from E. condamini occurred ~4.3 million years ago (Mya), while crown E. aquila diversification began ~0.9 Mya, coinciding with Pleistocene climate cycles. The Panama lineage diverged from the western slope–N Central cordillera ~0.4 Mya, suggesting post-Isthmus dispersal. Morphometric analyses of 69 museum specimens showed significant bill shape differentiation among regions, broadly consistent with genetic structure. Genetic distances among individuals were weakly correlated with both resistance and geographic (Euclidean) distances. Ecological niche projections indicated expanded connectivity during the mid-Holocene and LGM, but contraction and fragmentation during the LIG, with downward elevational shifts during colder periods (LGM and mid-Holocene) and upward shifts during warmer conditions (LIG and present). These models also revealed that ~227 500 km² of long-term climatic stability areas concentrated mostly in Amazonia and the Chocó. These results highlight recurrent isolation and reconnection of mountain environments as key drivers of foothill diversification and emphasize the importance of ecotonal taxa for understanding Andean evolutionary processes.