Allopatric speciation rather than ecological speciation drives evolution in an Amazonian fish.
Amazonia is characterized by very heterogeneous riverscapes dominated by two drastically divergent water types: black (ion-poor, dissolved organic carbonate rich and acidic) and white (nutrient rich and turbid) waters. Recent phylogeographic and genomic studies have associated the ecotone formed by these environments to ecologically driven speciation in fishes. With the objective of better understanding the evolutionary forces behind the Amazonian Teleostean diversification, we sampled 240 Mesonauta festivus from 12 sites on a wide area of the Amazonian basin. These sites included three confluences of black and white water environments to seek for repeated evidences of ecological speciation at these ecotones. Results obtained through our genetic assessment based on 41,268 SNPs contrast with previous findings and supports a low structuring power of water types. Conversely, we detected a strong pattern of isolation by unidirectional downstream water current and evidence of past events of vicariance potentially linked to the Amazon River formation. Using a combination of population genetic, phylogeographic analysis and environmental association models, we decomposed the spatial variance from the environmental genetic variance specifically to assess which evolutive forces have shaped inter-population differences in M. festivus’ genome. Our sampling design comprising four major Amazonian rivers and three confluences of black and white water rivers supports the possibility that past studies potentially confounded ecological speciation with a site effect unrepresentative of the full Amazonian watershed. While ecological speciation admittedly played a role in Amazonian fish species diversification, we argue that neutral evolutionary processes explain most of the divergence between M. festivus populations.