River erosion can shape fish evolution
The new findings could explain biodiversity hotspots in tectonically
quiet regions

Date:
May 25, 2023
Source:
Massachusetts Institute of Technology
Summary:
A new study of the freshwater greenfin darter fish suggests river
erosion can be a driver of biodiversity in tectonically inactive
regions.


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FULL STORY ==========================================================================
New findings could explain biodiversity hotspots in tectonically quiet
regions.

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.

* RELATED_TOPICS
o Plants_&_Animals
# Fish # Nature # Wild_Animals
o Earth_&_Climate
# Ecology # Earth_Science # Water
o Fossils_&_Ruins
# Evolution # Origin_of_Life # Early_Humans
* RELATED_TERMS
o Erosion o River o Floodplain o Biodiversity_hotspot o
Neon_tetra o Geology_of_the_Himalaya o Fishery o Channidae

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Maya F. Stokes, Daemin Kim, Sean F. Gallen, Edgar Benavides,
Benjamin P.

Keck, Julia Wood, Samuel L. Goldberg, Isaac J. Larsen, Jon
Michael Mollish, Jeffrey W. Simmons, Thomas J. Near, J. Taylor
Perron. Erosion of heterogeneous rock drives diversification
of Appalachian fishes. Science, 2023; 380 (6647): 855 DOI:
10.1126/science.add9791 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230525141356.htm

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