Conclusion
Our study aimed to investigate the support for the two main speciation
hypotheses (ecological and allopatric) used to explain the evolution ofM. festivus within the Amazon basin. We showed strong evidence
that the divergent physicochemical characteristics between black and
white water have a weak structuring power on M. festivuspopulations in the Amazonian watershed. Furthermore, our results
challenge the recently suggested ecological speciation hypothesis
explaining fish diversification in Amazonia. Unlike previous studies
focusing on a single confluence between black and white water, our
extensive sampling design comprising 12 populations of M.
festivus detected a genetic structure congruent with isolation by
unidirectional downstream water current, past geologic events, and
waterways connectivity shift. While the Brazilian Amazon supports one of
the richest fish faunas on Earth, our comprehension of the evolutionary
processes which shaped its biodiversity is still lacking. Understanding
the origin of such richness would help us protect its diversity. Our
study not only constitutes a step forward in understanding these
important processes but also provides a conceptual framework that should
benefit the sampling designs of future investigations on this matter.