How trees can drive fish speciation in the Amazon

New study tests Wallace’s intuition that water type can mediate fish diversification.

When I say ‘evolution’, you say ‘Darwin’. However, many people forget that the theory of evolution by natural selection was co-discovered by Alfred Russell Wallace. In fact, Darwin and Wallace presented their ideas in a joint paper entitled: “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.”


Wallace’s Intuition

Despite being less well-known than Darwin, Wallace contributed significantly to our current knowledge on evolution. For example, he identified a faunal divide in the Indonesian archipelago: the western part has animals of Asian origin, while the eastern part houses Australian animals. This dividing line is now known as the Wallace Line.

He also did fieldwork in the Amazon basin where he classified rivers based on their color and clarity: white, black and clear. When describing fish fauna in the Rio Negro (which is unsurprisingly a black river) he remarked that “Being a black-water river, most of its fishes are different from those found in the Amazon.

This suggestion – that fish speciation could be mediated by water type – was put to the test by Tiago Pires and colleagues in a recent paper in the Journal of Evolutionary Biology. They performed breeding experiments with the sailfin tetra (Crenuchus spilurus), a fish species that is comprised of two lineages: one restricted to the black Rio Negro and another that swims throughout several Amazonian white water rivers.



Two sailfin tetras in an aquarium (from:


Breeding experiments

The researchers recorded the reproductive success of 322 couples, trying out three different combinations: two Rio Negro fish, two Amazon fish or a mixed couple. All aquariums were filled with water from the Rio Negro river. The results were clear (see figure below), fish from the Rio Negro has a higher reproductive success compared to the other combinations. There seems to be some degree of reproductive isolation between the two lineages.

What could have caused the lower spawning rate in the Amazon fish? The researchers think that these fish probably suffered from physiological issues in the black water (remember that all aquariums were filled with Rio Negro water). This water has a lower pH compared to the clear Amazon water. Rio Negro fish are adapted to this pH, Amazon fish are not. It would be interesting to see the results if the couples were swimming in white water. Would the Rio Negro fish then show lower reproductive success?

graph pires.jpg

The outcome of the breeding experiments. Rio Negro fish show higher reproductive success compared to Amazon fish and mixed couples (from Pires et al. 2018)


What about the trees?

This study endorses Wallace’s intuition: Amazonian water type can mediate fish speciation. But what do trees have to do with all of this? The black waters are formed by incomplete decomposition of leaf litter from surrounding forests that grow on sandy soils. Hence, local forest and soil composition determine the water type of the river, which in turn drives fish diversification and speciation. So, indirectly trees drive fish speciation.



Pires, T.H., Borghezan, E.A., Machado, V.N., Powell, D.L., Ropke, C.P., Oliveira, C., Zuanon, J. & Farias, I.P. (2018) Testing Wallace’s intuition: water type, reproductive isolation and divergence in an Amazonian fish. Journal of Evolutionary Biology, 31:882-892.

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