Isolation in motion: how riverscape bridges and barriers shape the evolution of plants in fast-flowing river archipelagos

Islands serve as natural laboratories for studying evolution. In
fast-flowing rivers, rapids and waterfalls are discrete rocky habitats,
acting as insular freshwater landscape units. In the Neotropics, these
riverine archipelagos are inhabited one group of angiosperms, the
Podostemaceae. We used Marathrum foeniculaceum, a species distributed
across the Americas, to test ecological hypotheses derived from island
biogeography. Using whole‐genome and plastome data from individuals
collected across rivers in Panama and Colombia, we investigated the extent
and constraints to gene flow at multiple spatial scales using tests of
gene flow under the multispecies coalescent with migration model. We
further tested the hypothesis that gene flow is asymmetric and follows the
direction of river flow. Our results revealed strong genetic
differentiation among independent drainage basins and significant
isolation-by-distance, with overland distance posing a stronger barrier to
gene flow than distance along continuous river courses. Gene flow
identified between adjacent rivers likely reflects historical or episodic
river connections rather than frequent overland dispersals. Comparisons
between nuclear and plastid data indicate stronger constraints on seed
than pollen dispersal. Phylogeographic patterns in Podostemaceae further
suggest an east-to-west history of river connectivity across the Isthmus
of Panama. Together, these findings show that river connectivity and flow
directionality structure genetic variation in strictly riverine aquatic
angiosperms. Rapids and waterfalls act as evolutionary islands where
dispersal limitation, localized connectivity, and water flow interact to
shape population differentiation in riverine landscapes.