A recent genetic study tested the riverine barrier hypothesis in the Paraná–Paraguay River system.
How would you explain allopatric speciation to a layperson? Most biologists would roughly follow the definition you can find on Wikipedia: “A mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.” And perhaps they would add an example of a geographical barrier, such as a river. Studies on the Amazon river and its tributaries have indeed found support for this so-called riverine barrier hypothesis. But what about rivers in non-tropical regions? Subtropical bird species tend to show higher dispersal rates compared to their tropical cousins (see for example here). Hence, a river might not be such a formidable barrier for some subtropical species. A recent study in the journal Molecular Ecology put this idea to the test.
Cecilia Kopuchian and her colleagues studied the population genetics of seven pairs of subspecies that can be found around the subtropical Paraná–Paraguay River system. It concerns the following bird species:
- Green‐barred woodpecker (Colaptes melanochloros)
- Narrow‐billed woodcreeper (Lepidocolaptes angustirostris)
- Variable antshrike (Thamnophilus caerulescens)
- Rufous‐browed peppershrike (Cyclarhis gujanensis)
- Sayaca tanager (Thraupis sayaca)
- Red‐crested finch (Coryphospingus cucullatus)
- Ultramarine grosbeak (Cyanocompsa brissonii)
If the riverine hypothesis holds true in the subtropics, we would expect to find a clear genetic break between populations east and west of the rivers. Moreover, the timing of divergence between these pairs of populations should be similar across the seven bird species.
The genetic analyses revealed some surprising patterns. Only one species – the variable antshrike – followed the expected distribution with a clear genetic intergradation zone across the river (see panel a in figure below). In four other species, the genetic break between eastern and western populations was located between 150 and 300 kilometres east of the river (panels b-e), while in one species this break could be found on the western side of the rivers (panel f). Finally, the seventh species – the red‐crested finch – showed no relationship with the rivers whatsoever.
In addition to these discordant geographical patterns, the timing of divergence was widely different for the seven species. The authors report that “substantial variation in the number of generations since the split between individuals belonging to different genetic populations, ranging from 0.59 million generations in Coryphospingus cucullatus [red‐crested finch] to 2.25 million generations in Cyclarhis gujanensis [rufous‐browed peppershrike].” It is not looking good for the riverine barrier hypothesis here…
Geology and Ecotones
What can explain these discordant patterns? The researchers offer several possibilities. First, the geological history of the Paraná–Paraguay River system might provide a clue. The Paraná river has not always been in its current location. Geological studies revealed that this river has changed its course several times during the last few million years. Perhaps the genetic breaks uncovered in this study coincide with previous locations of the Paraná river.
Second, the location of the genetic breaks might be due to local ecological conditions, specifically the transition between different ecoregions (i.e. ecotones). Indeed, other studies have indicated that ecological gradients can drive genetic differentiation (interested readers can check these blog posts on saltmarsh sparrows or little greenbuls). In this case, it concerns the transition between the Chaco and Espinal ecoregions. Denser sampling of these regions is needed to confirm this possibility.
Taking into account the geological history and ecology of the area might still indicate that the genetic patterns are partly driven by rivers. The riverine barrier hypothesis is not dead yet in the subtropics.
Kopuchian, C., Campagna, L., Lijtmaer, D. A., Cabanne, G. S., García, N. C., Lavinia, P. D., … & Di Giacomo, A. S. (2020). A test of the riverine barrier hypothesis in the largest subtropical river basin in the Neotropics. Molecular Ecology.