The most important information in this study is that biodiversity mainly exists in areas of tectonic upheaval, such as the Himalayas and the Andes, which are particularly rich in flora and fauna due to their varying landscapes. Biodiversity, however, can thrive in geologically quieter locations where tectonics have not disturbed Earth for millennia.
The Appalachian Mountains are a good example: The range has witnessed little tectonic activity for hundreds of millions of years, but it is a major hotspot of freshwater species.
An MIT study discovered a geological process that can influence species diversity in tectonically inactive areas.
The researchers reveal in an article published today in Science that river erosion can boost biodiversity in these older, quieter ecosystems.
They found that as rivers eroded through various rock types in the southern Appalachians, a species of fish known as the greenfin tuna was floated into several tributaries of the river network.
These divided populations evolved over time into their different lineages. The researchers believe erosion caused the greenfin tuna to diversify.
While the separated populations appear identical on the surface, with the Green-finned darter’s distinctive green-tinged fins, their genetic makeup differs significantly. For now, the split populations are classified as a single species.
Maya Stokes, Ph.D. ’21, who performed some of the work as a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS), said: “Give this erosion process more time, and I think these separate lines will become different species.”
The researchers believe that erosion has caused many additional species to diversify into the watershed and other tectonically inactive locations around the planet.
“If we can understand the geological factors that contribute to biodiversity, we can better conserve it.” said Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT.
New studies emerged from Stokes’ doctoral work at MIT, where she and Perron explored the links between geomorphology and biology. Geomorphology is the study of how landscapes change over time. They came across Thomas Near’s research at Yale, where he looked at the lineage of freshwater fish in North America.
To show how and when specific species evolved and diverged from each other, Near analyzes DNA sequence data obtained from freshwater fish in different parts of North America.
The study found that the greenfin tuna were only found in the southern part of the Tennessee River basin, and that the genetic makeup of the populations varied based on the tributary in which they were found.
Stokes collected tissue samples of Groenfin darter from Near’s extensive Yale collection and from the field with help from TVA colleagues to investigate the causes of this pattern. She then compared the genes of each fish to every other fish in the dataset after analyzing DNA sequences from across the genome.
The team then built a phylogenetic tree of the green-fin darter based on genetic similarities between species.
Based on this tree, they found that fish in one tributary were more closely associated than fish in other rivers. In addition, fish from nearby streams were more similar than fish from more distant branches.
platform said, “Could there be a geological mechanism that over time has taken this single species and splintered into several, genetically distinct groups?”
Stokes and Perron then used genetic similarities to form a phylogenetic tree of the greenfin tuna using DNA sequences from across the genome.
They found that fish in a tributary were more closely related than fish from other branches, and fish in neighboring branches were more similar than fish from more distant rivers. They also found that rivers flowing through metamorphic rock are steeper and narrower, resulting in more turbulence, which greenfin arters apparently love.
The team wondered if the distribution of the greenfin darter’s habitat might have been shaped by a changing landscape of rocky nature 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 erod through different rock types.
They noted where and when connections between streams transitioned into non-metamorphic rock, preventing fish from moving between those tributaries. They created a history of these stumbles and compared it to the evolutionary tree of diverging greenfin darters.
The fish seemed to create separate lines in the same order as when their respective tributaries separated from the others.
“It means it’s likely that erosion through different rock layers caused isolation between different populations of the greenfin tuna and caused lineages to diversify.” Stokes said.
Josh Roering, a professor of Earth sciences at the University of Oregon, who was not involved in the study, said: “This study is very compelling because it reveals a much more subtle but powerful mechanism for speciation in passive margins. Stokes and Perron have revealed some of the intimate connections between aquatic species and geology that are far more common than we realize.”
This study was funded by the mTerra Catalyst Fund and the US National Science Foundation through the AGeS Geochronology Program.
Magazine reference:
- Maya F. Stroke, Sean F. Gallen, et al. Erosion of heterogeneous rock drives fish diversification in the Appalachians. Science. DOI: 10.1126/science.add9791
