Fish evolution and biodiversity is shaped by river erosion

Have you ever wondered what would happen if we could rewind and replay the evolution of species on our planet over hundreds of millions of years? 

It’s clear that biodiversity would cluster around areas teeming with tectonic upheaval. Regions like the Himalayan and Andean mountains, characterized by their constantly shifting landscapes, are particularly abundant in diverse flora and fauna. This tectonic turmoil diversifies species over time.

Yet, biodiversity is not exclusively tied to tectonic chaos. Take, for example, the Appalachian Mountains. Despite experiencing little tectonic activity for hundreds of millions of years, this region is a prominent hub of freshwater biodiversity. Why is this so?

Recent research from MIT provides an answer. The experts suggest that river erosion shapes the evolution and diversity of species in these tectonically dormant regions. The findings highlight how erosion drives biodiversity in these aged, tranquil environments.

Where the study was conducted

The researchers used the southern Appalachians, specifically the Tennessee River Basin, as a case study. This region is renowned for its tremendous diversity of freshwater fishes. 

As the team delved into the area, they found that rivers carving through varying types of rock changed the landscape, pushing a fish species – the greenfin darter – into different river tributaries. Over time, these isolated populations evolved into distinct lineages.

These findings lead the team to believe that erosion played a crucial role in the diversification of the greenfin darter. Despite outward similarities, like the green-tinged fins characteristic of the species, significant genetic differences exist among the separate populations. However, these diverse populations are still classified as a single species.

“Give this process of erosion more time, and I think these separate lineages will become different species,” said Maya Stokes, a former graduate student at MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).

The greenfin darter might not be the only species influenced by river erosion. The researchers speculate that erosion could be a driving force behind the diversification of several species within the basin and perhaps even in other tectonically inactive regions worldwide.

“If we can understand the geologic factors that contribute to biodiversity, we can do a better job of conserving it,” said study co-author Professor Taylor Perron.

The study also includes contributions from Yale University, Colorado State University, the University of Tennessee, the University of Massachusetts at Amherst, and the Tennessee Valley Authority (TVA).

Origins of the hypothesis

The genesis of this study lies in Stokes’ PhD work at MIT, where she collaborated with Professor Perron to study connections between geomorphology (the study of how landscapes evolve) and biology. They came across Thomas Near’s work at Yale. He studies North American freshwater fish lineages using DNA sequence data to unravel the evolutionary divergence and connections between different species.

Near’s research led to an intriguing observation. The habitat distribution map of the greenfin darter indicated that the species was found exclusively in the southern half of the Tennessee River Basin. Moreover, mitochondrial DNA sequence data showed a distinct genetic makeup among the fish populations based on the tributary they inhabited.

To understand this pattern, Stokes examined greenfin darter tissue samples from Near’s collection at Yale and from fieldwork conducted with TVA colleagues. By analyzing DNA sequences and comparing the genes of individual fish, they constructed a phylogenetic tree based on genetic similarities between fish. 

What the researchers learned

The results revealed that fish within a tributary were more genetically related to each other than to fish in other tributaries. This led to a question: could a geological mechanism be responsible for splintering this single species into different, genetically distinct groups over time?

Investigations revealed a strong correlation between the habitats of the greenfin darter and the type of rock where they were found. Notably, most of the southern half of the Tennessee River Basin, where the species thrive, is composed of metamorphic rock, whereas the northern half predominantly features sedimentary rock, absent of the species.

Rivers flowing through metamorphic rock were found to be steeper and narrower, creating more turbulence – a condition greenfin darters seem to favor. This led to a question: Could the landscape changes resulting from river erosion into varying rock types over time have shaped the distribution of the greenfin darter’s habitat?

Creating a simulated landscape model

To validate this hypothesis, the researchers designed a model to simulate landscape evolution due to river erosion through different rock types. They fed the model with information about current rock types in the Tennessee River Basin. The simulation was then run backward to depict the region millions of years ago, when more metamorphic rock was exposed.

Running the model forward showed the shrinking exposure of metamorphic rock over time. They noted where and when tributary connections crossed into non-metamorphic rock, effectively creating barriers for fish movement between these tributaries.

The scientists then sketched a timeline of these blocking events and compared it to the phylogenetic tree depicting the divergence of greenfin darters. The results were strikingly similar: the fish seemed to form distinct lineages in the same order as when their respective tributaries became isolated.

“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,” said Stokes.

This groundbreaking study was supported by the Terra Catalyst Fund and the U.S. National Science Foundation through the AGeS Geochronology Program and the Graduate Research Fellowship Program. Stokes’ work at MIT was supported through the Martin Fellowship for Sustainability and the Hugh Hampton Young Fellowship. 

The research, which is published in the journal Science, not only reveals the influence of geologic factors on biodiversity but also underscores the need for understanding these processes to conserve the rich tapestry of life on our planet.

More about river erosion

River erosion is a natural process that involves the wearing away of the Earth’s surface by the action of flowing water. This process can shape the land in various ways, leading to a number of distinct features.

River erosion primarily occurs through four processes: hydraulic action, abrasion, attrition, and corrosion (or solution).

Hydraulic action

This process occurs when the force of water removes soil and rock from the river bed and banks. It is especially effective when water enters cracks and crevices, causing pressure to build up and pieces of rock to break off.

Abrasion

Also known as corrasion, this is the process by which rocks and sediment carried by the river scrape and wear away the bed and banks. Essentially, the river bed and banks are ‘sand-papered’ by the river’s load.

Attrition

This process involves the wearing down of the load (rocks and pebbles) carried by the river itself. When these materials collide, they break into smaller pieces and become smoother. Over time, large boulders can be reduced to tiny particles of sand and silt through attrition.

Corrosion (or solution)

This process involves the river water dissolving certain types of rocks. For instance, limestone and chalk can be gradually dissolved by the carbonic acid present in water.

These processes of river erosion contribute to the formation of various landforms such as v-shaped valleys, waterfalls, meanders, and oxbow lakes.

In a v-shaped valley, river erosion creates a steep-sided, narrow valley in its upper course. When a river flows over an area with layers of hard and soft rock, a waterfall may form as the softer rock erodes more quickly, causing the river to drop steeply.

As a river moves towards its middle and lower courses, lateral (sideways) erosion becomes more significant, which can lead to the formation of large bends known as meanders. If a meander continues to erode laterally, it can eventually form a separate body of water called an oxbow lake.

River erosion is a significant factor in the overall process of the water cycle and sediment transportation. It’s important in shaping the Earth’s surface and creating habitats for various forms of life. 

However, excessive river erosion can also have negative impacts, such as loss of land, property damage, and negative effects on ecosystems. Therefore, managing river erosion is often a significant consideration in environmental and urban planning.

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