A recent study tested several hypotheses with genetic data.
The Amazonian floodplains are home to numerous endemic species. Biologists have been cataloging this amazing biodiversity for centuries, but the processes that generated this mind-boggling variety in species remain a matter of debate. In a recent Journal of Biogeography study, Leilton Luna and his colleagues focused on the Striped Woodcreeper (Xiphorhynchus obsoletus) to gain more insights into the diversification processes in the Amazonian floodplains. Using a genetic dataset of more than 2000 markers, the researchers tested several hypotheses to explain the population genetic structure of this widespread generalist.
Ecological Gradients
Let’s have look at the different hypotheses. The null hypothesis concerned a pattern of isolation-by-distance (IBD): genetic differentiation increases with geographical distance. No other processes, such as natural selection, are needed to explain the genetic patterns. Given the large variety in habitats, this hypothesis seems unlikely, but it is always important to include a null hypothesis.
The second hypothesis focused on the ecological gradients along the Amazonian floodplains, specifically the properties of the river water. The authors explained the situation nicely in the introduction of their paper:
Sediment-rich white water rivers (i.e., várzea) with Andean sedimentary origin, and black and clear water rivers (i.e., igapó), poor in suspended sediments, and influenced by cratonic soils of the Brazilian and Guiana Shields, support very distinct floodplains in different parts of Amazonia, with várzeas distributed mainly in the west, and igapós mostly in the central and eastern parts of the basin.
If the ecological gradient hypothesis holds true, we would expect that the genetic population structure of the Striped Woodcreeper coincides with the different river types.
Climate Change
The remaining two hypotheses considered the climatic changes during the Quaternary (from 2.5 million years ago until the present day). The first hypothesis stated that increased rainfall after the last glaciation allowed the flooded habitats of western Amazonia to spread eastwards. The newly available habitats would provide additional habitats for bird species that would consequently also expand to the east.
The second climate-related hypothesis put more emphasis on the cycles of sedimentation across the Amazonian basin. The alternating processes of deposition and removal of sediments (driven by large-scale precipitations patterns) could change the connectivity between the flooded habitats and their inhabitants. These sedimentation processes might impact genetic population structure by isolating or connecting particular populations along the floodplains.
Gene Flow
Four clear hypotheses with accompanying predictions. But what did the researchers find? The genetic analyses uncovered three main clusters, namely “populations distributed respectively in the western (Solimões, Japurá, Branco, Negro—white, clear and black waters), central (Amazonas and Tapajós—white and clear waters), and eastern (Xingu and Araguaia—clear waters) Amazon Basin.” From this description, you can deduce that these three populations are not associated with one river type. Based on this finding, we can thus reject the ecological gradient hypothesis.
The three populations did not show a clear pattern of isolation-by-distance. An estimated effective migration surface (EEMS) analysis suggested two major barriers to gene flow (i.e. the orange areas in the left figure below). Although there were signs of IBD on a regional scale, this neutral pattern could not be extended across the floodplains. The IBD hypothesis can also be rejected.
Sedimentation Cycles
We can now assess the climate-related hypotheses. As explained above, one scenario involved eastward population expansion due to increased rainfall after the last glaciation. Colonization of new areas is mostly associated with a decrease in genetic diversity (as only a subset of the population will expand). This expected genetic pattern did not emerge from the analyses. Instead of decreased genetic diversity towards the east, the researchers found higher genetic diversity in regions where two populations mixed. No support for the “rainfall-with-eastward-expansion” hypothesis.
And that brings us to the final hypothesis: sedimentation cycles that drive population isolation and connectivity. This scenario was supported by demographic modelling of past population changes. The researchers noted that “differentiation among populations was centered in the Late-Pleistocene and is best explained by partial isolation, probably due to changes in the connectivity of floodplain habitats due to large-scale climate shifts across the Amazon Basin.” The geographic barriers in the EEMS coincide with regions that experienced high sedimentary activity in the past, suggesting a central role for sedimentation cycles. Whether this scenario applies to other bird species in the Amazonian floodplains remains an open question.
References
Luna, L. W., Ribas, C. C., & Aleixo, A. (2022). Genomic differentiation with gene flow in a widespread Amazonian floodplain‐specialist bird species. Journal of Biogeography, 49(9), 1670-1682.
Featured image: Striped Woodcreeper (Xiphorhynchus obsoletus) © Nick Athanas | Flickr
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