[190 words] 33not peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/106302 doi: bioRxiv preprint first posted online Feb. 6, 2017; 2• The Andean mountains of South America are the most species-rich 34 biodiversity hotspot worldwide with about 15% of the world's plant species, in 35 only 1% of the world's land surface. Orchids are a key element of the Andean 36 flora, and one of the most prominent components of the Neotropical epiphyte 37diversity, yet very little is known about their origin and diversification. 38• We address this knowledge gap by inferring the biogeographical history and 39 evolutionary dynamics of the two largest Neotropical orchid groups 40 (Cymbidieae and Pleurothallidinae), using two unparalleled, densely-sampled 41 orchid phylogenies (including 400+ newly generated DNA sequences), 42 comparative phylogenetic methods, geological and biological datasets.

The Andean uplift is one of the major orographic events in the New World and has impacted considerably the diversification of numerous Neotropical organisms. Despite its importance for biogeography, the specific role of mountain ranges as a dispersal barrier between South and Central American lowland plant lineages is still poorly understood. The swan orchids (Cycnoches) comprise ca 34 epiphytic species distributed in lowland and pre-montane forests of Central and South America. Here, we study the historical biogeography of Cycnoches to better understand the impact of the Andean uplift on the diversification of Neotropical lowland plant lineages. Using novel molecular sequences (five nuclear and plastid regions) and twelve biogeographic models with and without founder-event speciation, we infer that the most recent common ancestor of Cycnoches may have originated in Amazonia ca 5 Mya. The first colonization of Central America occurred from a direct migration event from Amazonia, and multiple bidirectional trans-Andean migrations between Amazonia and Central America took place subsequently. Notably, such biological exchange occurred well after major mountain building periods. The Andes have not acted as an impassable barrier for epiphytic lowland lineages such as orchids having a great potential for effortless dispersal because of the very light, anemochorous seeds.