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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    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   
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   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   
      
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   Story Source: Materials provided by   
   Massachusetts_Institute_of_Technology. Original written by Jennifer   
   Chu. Note: Content may be edited for style and length.   
      
      
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   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   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230525141356.htm   
      
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