A test of the riverine barrier hypothesis in the largest subtropical river basin in the Neotropics.

The riverine barrier hypothesis proposes that large rivers represent geographical barriers to gene flow for terrestrial organisms, leading to population differentiation and ultimately allopatric speciation. Here we assess for the first time if the subtropical Paraná-Paraguay River system in the Del Plata basin, second in size among South American drainages, acts as a barrier to gene flow for birds. We analysed the degree of mitochondrial and nuclear genomic differentiation in seven species with known subspecies divided by the Paraná-Paraguay River axis. Only one species showed genetic differentiation concordant with the current river channel, but another five species have an east/west genetic split broadly coincident with the Paraná River's dynamic palaeochannel, suggesting this fluvial axis has had a past role in shaping present-day genetic structure. Moreover, dating analyses show that these splits have been asynchronous, with species responding differently to the riverine barrier. Comparisons informed by the geological history of the Paraná River and its influence on the ecological and climatic differences among ecoregions in the study area further bolster the finding that responses to this geographical barrier have been species-specific.