Andean uplift, drainage basin formation, and the evolution of plants living in fast-flowing aquatic ecosystems in northern South America.

Northern South America is a geologically dynamic and species-rich region. Fossil and stratigraphic data show that mountain uplift in the tropical Andes reconfigured river drainages. These landscape changes shaped the evolution of the flora in the region, yet the impacts on aquatic taxa have been overlooked. We explore the role of landscape change on the evolution of plants living strictly in rivers across drainage basins in northern South America by conducting population structure, phylogenetic inference, and divergence-dating analyses for two species in the genus Marathrum (Podostemaceae). Mountain uplift and drainage basin formation isolated populations of M. utile and M. foeniculaceum in northern South America and created barriers to gene flow across river drainages. Sympatric species hybridize and the hybrids show the phenotype of one parental line. We propose that the pattern of divergence of populations reflects the formation of river drainages, which was not complete until < 4.1 million yr ago (Ma). Our study provides a clear picture of the role of landscape change on the evolution of plants living strictly in rivers in northern South America. By shifting the focus to aquatic taxa, we provide a novel perspective on the processes shaping the evolution of the Neotropical flora.