Fish Evolution Shaped by Intense Rapids

Cichlids Telegramma brichardi

Photo credit: Oliver Lucanus.
Source: American Museum of Natural History.

Congo River Fish Evolution Shaped by Intense Rapids | American Museum of Natural History.

The cichlids are a large group of fish known for their ability to rapidly adapt, diverge and speciate in response to environmental changes as they did in the East African lakes when presented with a new range of potential ecological niches to fill.

In this paper a team led by scientists at the American Museum of Natural History, the City University of New York, and Fordham University, have shown that rock-dwelling cichlids of the genus Teleogramma are diversifying in a very sort stretch near the mouth of the Congo River in Africa. The final 200-mile stretch of the Congo River is exceptional in depth, speed, and turbulence, forming the world’s most extreme rapids.

In this very short section of the Congo, we find a tremendous diversity of fishes. We also know that this part of the river is relatively young, originating only about 3 to 5 million years ago. So what is it about this system that makes it such a pump for species?

Melanie Stiassny, Co-author.
Axelrod Research Curator, American Museum of Natural History Department of Ichthyology.

This stretch is also known for its biodiversity with more than 300 species of fish having been identified. It is also known to be only between three and five million years old.

This diversity has long been a puzzle for evolutionary biologists because there are few physical barriers to isolate populations and prevent gene transfer between populations.

To understand how this diversity arose, the team analysed the DNA of Teleogramma cichlids and showed that even populations living less than a mile apart exchange very few genes. 

Abstract

[Regrettably, the copyright holders, John Wiley & Sons Ltd. felt unable to grant permission for the abstract to be reprinted here. It is available via the following link.]

Alter, S. E., Munshi-South, J. and Stiassny, M. L. J. (2017),
Genomewide SNP data reveal cryptic phylogeographic structure and microallopatric divergence in a rapids-adapted clade of cichlids from the Congo River.
Mol Ecol. doi:10.1111/mec.13973



The genetic separation between these fishes show that the rapids are working as strong barriers, keeping them apart. What's particularly unique about the lower Congo is that this diversification is happening over extremely small spatial scales, over distances as small as 1.5 kilometers. There is no other river like it.

Elizabeth Alter. Lead author
The City University of New York’s Graduate Center and York College.

This adds weight to the hypothesis that it is the hydrodynamics of the rapids that are causing this isolation; fish cannot pass from one small area to another because currents are just too strong. They become genetically isolated and evolve in a direction determined by the local micro-environment.

Note that the mystery here was not how this diversification came about despite the lack of geographical isolation but what it was that caused the geographical isolation into diverging gene-pools in the first place. There was never any assumption that the ToE was somehow inadequate to explain the diversification of cichlids, and very probably fish of other groups. The question was always about how exactly the necessary conditions had arisen. So much for a theory in crisis, about to be overthrown by one involving magic.

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