Study: River erosion can shape fish evolution

If we could turn the tape of the evolution of species around the world and run it forward over hundreds of millions of years to the present day, we would see clusters of biodiversity around areas of tectonic upheaval. Tectonically active regions such as the Himalayas and the Andes are particularly rich in flora and fauna due to their changing landscapes, which serve to divide and diversify species over time.

But biodiversity can also thrive in some geologically calmer regions, where tectonics hasn’t shaken the Earth for millennia. The Appalachians are a prime example of this: the range hasn’t seen much tectonic activity in hundreds of millions of years, yet the region is a notable hotspot for freshwater biodiversity.

Now, an MIT study identifies a geological process that may shape species diversity in tectonically inactive regions. in the paper appearing in Sciencesresearchers report that river erosion can be a driver of biodiversity in these older, quieter environments.

They make their case in the southern Appalachians, specifically the Tennessee River basin, an area known for its huge diversity of freshwater fish. The team found that as rivers eroded through the different rock types in the area, the changing landscape pushed a type of fish known as green-fin sedge into different tributaries of the river network. Over time, these separate populations have evolved into their own distinct lineages.

The team speculates that erosion may have prompted greenfinches to diversify. Although the separate populations appear to be outwardly similar, with the distinctive green-finned fin-fin fins, they differ fundamentally in their genetic make-up. At the moment, the separated groups are classified as a single species.

“Give this erosion process a little more time, and I think these separate lineages will become different species,” says Maia Stokes, Ph. (EAPS).

The bluefin may not be the only species diversifying as a result of river erosion. The researchers believe that erosion may have prompted many other species to diversify throughout the basin, and possibly other tectonically inactive regions around the world.

“If we can understand the geological factors that contribute to biodiversity, we can do a better job of preserving it,” says Taylor Perron, MIT Professor of Earth, Atmospheric and Planetary Sciences, Cecil and Ida Green.

Study co-authors include collaborators at Yale University, Colorado State University, the University of Tennessee, the University of Massachusetts Amherst, and the Tennessee Valley Authority (TVA). Stokes is currently an assistant professor at Florida State University.

fish in the trees

The new study grew out of Stokes’ doctoral work at MIT, where she and Peron were exploring connections between geomorphology (the study of how landscapes evolve) and biology. They came across work at Yale University by Thomas Near, who studies the lineages of freshwater fish in North America. Near uses DNA sequence data collected from freshwater fish across different regions of North America to show how and when specific species evolved and diverged in relation to each other.

Near brought an intriguing note to the team: A habitat distribution map for greenfinches shows that the fish was found in the Tennessee River basin—but only in the southern half. Moreover, Near had mitochondrial DNA sequencing data showing that the fish populations looked different in their genetic makeup depending on which tributary they were found in.

To investigate the reasons for the pattern, Stokes collected green-fin soft tissue samples from Yale’s extensive Nier collection, as well as from the field with the help of her colleagues at TVA. Then she analyzed DNA sequences across the entire genome, and compared the genes of each individual fish to every other fish in the data set. The team then created a phylogenetic tree of the green-finned dab, based on genetic similarity between the fish.

From this tree, they noticed that fish within one tributary were more associated with each other than with fishing in the other tributaries. Moreover, fish within adjacent tributaries were more similar to each other than fish from more distant tributaries.

“Our question was, could there be a geological mechanism that, over time, has taken this individual species, and divided it into different, genetically distinct populations?” Peron says.

A changing landscape

Stokes and Perone began by noticing a “close correlation” between the habitats of greenfin jays and the type of rocks they are found on. In particular, the southern half of the Tennessee River Basin, where the species abound, is composed of metamorphic rocks, while the northern half is composed of sedimentary rocks, where fish are not found.

They also note that rivers passing through metamorphic rocks are steeper and narrower, generally creating more turbulence, and the distinctive green-finned fins seem to favor them. The team wondered: Could the greenfin habitat distribution have been shaped by changing rock-type landscapes, as rivers eroded into the land over time?

To test this idea, the researchers developed a model to simulate how landscapes evolve as rivers erode through different rock types. They fed model information about the rock types in the Tennessee River basin today, then ran the simulation again to see what the same area might have looked like millions of years ago, when more metamorphic rocks were exposed.

Then they ran the model forward and observed how the exposure of the metamorphic rocks diminished over time. They took special notes of where and when the links between tributaries crossed into non-metamorphic rock, preventing fish from passing between those tributaries. They made a simple timeline of these blocking events and compared it to the phylogenetic tree of green forked-finger quadrants. The two were remarkably similar: the fish seem to form separate lineages in the same order as when their tributaries separated from the others.

“This means that it is plausible that erosion through different rock layers caused isolation between different groups of greenfinger patches and caused diversification of lineages,” Stokes says.

This research was supported in part by the Terra Catalyst Fund and the US National Science Foundation through the AGeS Geochronology and Graduate Research Fellowship Program. While at MIT, Stokes was supported by a Martin Sustainability Fellowship and a Hugh Hampton Young Fellowship.

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By Jennifer Chu, MIT News Desk