River erosion can shape fish evolution, study suggests
The new findings could explain biodiversity hotspots in tectonically quiet regions
Peer-Reviewed PublicationIMAGE: TAYLOR PERRON AND MAYA STOKES SAMPLE STREAM SEDIMENTS. “IF WE CAN UNDERSTAND THE GEOLOGIC FACTORS THAT CONTRIBUTE TO BIODIVERSITY, WE CAN DO A BETTER JOB OF CONSERVING IT,” SAYS PERRON. view more
CREDIT: IMAGE: SEAN GALLEN
If we could rewind the tape of species evolution around the world and play it forward over hundreds of millions of years to the present day, we would see biodiversity clustering around regions of tectonic turmoil. Tectonically active regions such as the Himalayan and Andean mountains are especially rich in flora and fauna due to their shifting landscapes, which act to divide and diversify species over time.
But biodiversity can also flourish in some geologically quieter regions, where tectonics hasn’t shaken up the land for millennia. The Appalachian Mountains are a prime example: The range has not seen much tectonic activity in hundreds of millions of years, and yet the region is a notable hotspot of freshwater biodiversity.
Now, an MIT study identifies a geological process that may shape the diversity of species in tectonically inactive regions. In a paper appearing in Science, the researchers report that river erosion can be a driver of biodiversity in these older, quieter environments.
They make their case in the southern Appalachians, and specifically the Tennessee River Basin, a region known for its huge diversity of freshwater fishes. The team found that as rivers eroded through different rock types in the region, the changing landscape pushed a species of fish known as the greenfin darter into different tributaries of the river network. Over time, these separated populations developed into their own distinct lineages.
The team speculates that erosion likely drove the greenfin darter to diversify. Although the separated populations appear outwardly similar, with the greenfin darter’s characteristic green-tinged fins, they differ substantially in their genetic makeup. For now, the separated populations are classified as one single species.
“Give this process of erosion more time, and I think these separate lineages will become different species,” says Maya Stokes PhD ’21, who carried out part of the work as a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).
The greenfin darter may not be the only species to diversify as a consequence of river erosion. The researchers suspect that erosion may have driven many other species to diversify throughout the basin, and possibly other tectonically inactive regions around the world.
“If we can understand the geologic factors that contribute to biodiversity, we can do a better job of conserving it,” says Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric, and Planetary Sciences at MIT.
The study’s co-authors include collaborators at Yale University, Colorado State University, the University of Tennessee, the University of Massachusetts at Amherst, and the Tennessee Valley Authority (TVA). Stokes is currently an assistant professor at Florida State University.
Fish in trees
The new study grew out of Stokes’ PhD work at MIT, where she and Perron were exploring connections between geomorphology (the study of how landscapes evolve) and biology. They came across work at Yale by Thomas Near, who studies lineages of North American freshwater fishes. Near uses DNA sequence data collected from freshwater fishes across various regions of North America to show how and when certain species evolved and diverged in relation to each other.
Near brought a curious observation to the team: a habitat distribution map of the greenfin darter showing that the fish was found in the Tennessee River Basin — but only in the southern half. What’s more, Near had mitochondrial DNA sequence data showing that the fish’s populations appeared to be different in their genetic makeup depending on the tributary in which they were found.
To investigate the reasons for this pattern, Stokes gathered greenfin darter tissue samples from Near’s extensive collection at Yale, as well as from the field with help from TVA colleagues. She then analyzed DNA sequences from across the entire genome, and compared the genes of each individual fish to every other fish in the dataset. The team then created a phylogenetic tree of the greenfin darter, based on the genetic similarity between fish.
From this tree, they observed that fish within a tributary were more related to each other than to fish in other tributaries. What’s more, fish within neighboring tributaries were more similar to each other than fish from more distant tributaries.
“Our question was, could there have been a geological mechanism that, over time, took this single species, and splintered it into different, genetically distinct groups?” Perron says.
A changing landscape
Stokes and Perron started to observe a “tight correlation” between greenfin darter habitats and the type of rock where they are found. In particular, much of the southern half of the Tennessee River Basin, where the species abounds, is made of metamorphic rock, whereas the northern half consists of sedimentary rock, where the fish are not found.
They also observed that the rivers running through metamorphic rock are steeper and more narrow, which generally creates more turbulence, a characteristic greenfin darters seem to prefer. The team wondered: Could the distribution of greenfin darter habitat have been shaped by a changing landscape of rock type, as rivers eroded into the land over time?
To check this idea, the researchers developed a model to simulate how a landscape evolves as rivers erode through various rock types. They fed the model information about the rock types in the Tennessee River Basin today, then ran the simulation back to see how the same region may have looked millions of years ago, when more metamorphic rock was exposed.
They then ran the model forward and observed how the exposure of metamorphic rock shrank over time. They took special note of where and when connections between tributaries crossed into non-metamorphic rock, blocking fish from passing between those tributaries. They drew up a simple timeline of these blocking events and compared this to the phylogenetic tree of diverging greenfin darters. The two were remarkably similar: The fish seemed to form separate lineages in the same order as when their respective tributaries became separated from the others.
“It means it’s plausible that erosion through different rock layers caused isolation between different populations of the greenfin darter and caused lineages to diversify,” Stokes says.
This research was supported, in part, by the Terra Catalyst Fund and the U.S. National Science Foundation through the AGeS Geochronology Program and the Graduate Research Fellowship Program. While at MIT, Stokes received support through the Martin Fellowship for Sustainability and the Hugh Hampton Young Fellowship.
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Written by Jennifer Chu, MIT News Office
JOURNAL
Science
ARTICLE TITLE
Erosion of heterogeneous rock drives diversification of Appalachian fishes
ARTICLE PUBLICATION DATE
26-May-2023
River erosion drives fish biodiversity in the Appalachians
New Haven, Conn. — The gradual erosion of layers of rock by rivers flowing through the Appalachian Mountains generates biodiversity of freshwater fish species, suggests a new Yale-led study that offers insight into the causes of species richness in the ancient mountain range.
Researchers have previously associated high biodiversity in mountain ranges, including the Andes and Himalaya, with tectonic uplift — the shifting of plates in the Earth’s crust that forms mountains, plateaus, and other geologic structures — triggering environmental changes that create conditions ripe for species diversification. But this explanation does not account for the high biodiversity found in older mountain ranges, such as the species-rich Appalachians, where tectonic uplift ceased hundreds of millions of years ago.
For the new study, published May 26 in the journal Science, researchers analyzed populations of greenfin darters, Nothonotus chlorobranchius, a fish species only found in the upper Tennessee River system in the southern Appalachians, and the river basin’s underlying geology.
They found that river water has gradually eroded a top layer of metamorphic rock in portions of the upper Tennessee River basin, exposing softer sedimentary rock that acts as a barrier, isolating populations of the greenfin darter in river channels still flowing over metamorphic rock. As with the finch populations observed by Charles Darwin on the Galapagos Islands, such geographic isolation prevents the greenfin darters from breeding across populations, said Maya F. Stokes, the paper’s lead author, who conducted the research while a postdoctoral researcher in Yale’s Department of Ecology and Evolutionary Biology. This, she said, sets the stage for them to evolve separately from each other.
“We know that speciation happens when populations are geographically isolated, but it isn’t clear how isolation happens without dramatic geomorphological changes across the landscape,” Stokes, now an assistant professor of geology at Florida State University, said. “Our study shows that greenfin darter populations are being isolated through the gradual internal dynamics of erosion, not major external forces like climate change, glaciation, or tectonic activity.”
The upper Tennessee River basin is divided into the highland Blue Ridge geologic area composed of metamorphic rock and a lowland Valley and Ridge area composed of sedimentary rock. Metamorphic rocks form when existing rocks are subjected to environmental change, such as high heat, high pressure, or a combination of both and in this landscape are harder to erode than sedimentary rock. This makes the highland section steeper and more rugged than the lowland Valley and Ridge section. The greenfin darter populations mostly inhabit tributaries in the Blue Ridge section.
The researchers collected greenfin darter specimens from populations throughout the Blue Ridge tributaries. Their dataset also included samples from the Yale Peabody Museum’s tissue collection. Through genomic analysis of DNA sequence data, the researchers determined the evolutionary lineages of the geographically separated greenfin darter populations.
“The DNA sequencing found genetic variation among the separate populations on par with what we find between separate species,” said senior author Thomas J. Near, professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences. “We don’t delimit them as separate species in this study, and they show little variation in physical characteristics, but the genetic analysis suggests we’re seeing speciation in action. I think ultimately these lineages will become separate species if they aren’t already.”
“It’s possible, if not likely, that the process of erosion we identified is responsible for past speciation,” added Near, who is also the Bingham Oceanographic Curator of Ichthyology at the Yale Peabody Museum.
The researchers also compared the evolutionary history of the fish populations to the geologic history of the upper Tennessee River basin. They used a geometric model of bedrock erosion that shows how the exposure of metamorphic rock (where the greenfin darter is found) has shrunk over geologic time, while that of sedimentary rock has expanded. They suggest that this process reduced the habitat connectivity between tributaries, leading to the isolation of lineages residing in tributaries flowing over the remaining metamorphic rock.
“The basic concept here is that rivers erode away different kinds of rock exposing new kinds of rock that may affect the spatial distribution of suitable habitat,” said Stokes, who was a Gaylord Donnelley postdoctoral associate at the Yale Institute for Biospheric Studies.
Why sedimentary rock forms a barrier to the greenfin darters’ movement is unknown, but the researchers point out that different types of rock influence freshwater habitats in multiple ways, including flow velocity, water chemistry, and the amount of sediment suspended in the water.
The study was co-authored by Daemin Kim, Edgar Benavides, and Julia Wood of Yale’s Department of Ecology and Evolutionary Biology; Sean F. Gallen of Colorado State University; Benjamin P. Keck of the University of Tennessee, Knoxville; Samuel L. Goldberg and J. Taylor Perron of the Massachusetts Institute of Technology; Isaac J. Larsen of the University of Massachusetts, Amherst; and Jon Michael Mollish and Jeffrey W. Simmons of the Tennessee Valley Authority.
JOURNAL
Science
ARTICLE PUBLICATION DATE
26-May-2023
