The snail population in the Koster Islands, Sweden, is undergoing rapid evolutionary changes due to genetic diversity while facing environmental pressures.
In 1988, the Koster Islands (a group of islands off the western coast of Sweden near Norway) were impacted by an exceptionally dense bloom of toxic algae that devastated the marine snail population. One might wonder why the fate of snails on a small, three-square-meter rock in the open sea is of such importance. However, this event created a unique opportunity to predict and witness the evolutionary process unfolding before our eyes.
Previously, the islands and small coastal reefs were home to dense and diverse populations of the marine snail Littorina saxatilis. While snail populations on larger islands recovered within two to four years (at times, some populations dropped below 1%), certain populations on the reefs still struggled to recover after the devastation.
Marine snails in Sweden.
A Groundbreaking Experiment
Marine ecologist Kerstin Johannesson from the University of Gothenburg, Sweden, saw a unique opportunity. In 1992, she reintroduced the L. saxatilis snails back into their lost habitat and began an experiment with far-reaching implications more than 30 years later. This later experiment involved international collaboration from researchers at the Institute of Science and Technology Austria (ISTA), Nord University, Norway, the University of Gothenburg, Sweden, and the University of Sheffield, UK, aiming to observe the ongoing evolutionary process.
L. saxatilis is a common marine snail species found along the North Atlantic coast, where different populations evolve characteristics adapted to their environments. These characteristics include size, shell shape, shell color, and behavior. The differences between these distinct characteristics give rise to two branches: rock snails and wave snails. Wave snails are typically smaller, with thin shells featuring distinctive colors and patterns, larger round apertures, and often exhibit reckless behavior. In contrast, rock snails are larger, have thicker, unpatterned shells, smaller and elongated apertures, and exhibit more cautious behavior due to their environments being preyed upon by predators.
The Koster Islands in Sweden are home to both species of L. saxatilis, often found side by side on the same island or just a few hundred meters apart in the sea. Before the toxic algae bloom in 1988, wave snails resided on the reefs, while the nearby coast was a shared habitat for both rock and wave snails. This close spatial proximity would prove crucial.
Noticing that the wave snail population on the reefs had been completely wiped out due to the toxic algae, Johannesson decided in 1992 to restock rock snails into one of these reefs. With one to two generations per year, she was correct in expecting the rock snails to adapt to their new environment right before the scientists’ eyes. Diego Garcia Castillo, a graduate student at the Barton Group of ISTA and one of the initiating authors of the research, stated: “Our colleagues observed evidence of the snail’s adaptation within the first decade of the experiment.” Castillo added: “Throughout the 30 years of the experiment, we have been able to accurately predict what the snails would look like and which gene regions would be affected. The transformation process was rapid and powerful.”
Revisiting Evolutionary Traits
However, the snails did not evolve these characteristics from scratch. Co-author Anja Marie Westram, now a researcher at Nord University, explained: “Some genetic diversity was already present in the original rock snail population but at low frequencies. This is due to the species having experienced similar conditions in the recent past. The access to a large gene pool has propelled this rapid evolutionary process.”
The research team examined three aspects over the years of conducting the experiment: the phenotypes of the snails, the genetic variability of individual snails, and larger genetic changes affecting entire chromosome regions known as “chromosomal inversions.”
In less than 30 years, the shell of the original rock snail has evolved to resemble that of the wave snail.
In the first few generations, researchers witnessed an intriguing phenomenon known as “phenotypic plasticity”: Immediately after the “recolonization,” the snails changed shape to adapt to the new environment. However, the population also quickly began to change genetically. Researchers could predict the extent and direction of genetic changes, particularly concerning chromosomal inversions.
They demonstrated that the rapid and powerful changes in the snail species could be attributed to two complementary processes: One is the rapid selection of traits that were present but less expressed in the newly migrated rock snail population, and the other is gene flow from neighboring wave snails (just a few hundred meters away) that may drift into mating.
Evolution Under Environmental Pressure
Theoretically, scientists know that a species with sufficient genetic variation can adapt faster to changes. However, few studies aim to experimentally test the evolutionary process over time in nature. Garcia Castillo stated: “This work allows us to take a closer look at the repeatable evolutionary process and predict how a population might develop characteristics that have evolved distinctly in the past under similar conditions.”
The research team now wants to understand how species can adapt to modern environmental challenges such as pollution and climate change. Westram noted: “Not all species can access large gene pools, and developing new traits from scratch is a slow and tedious process. The adaptation process is very complex, and our planet is also facing unpredictable changes due to extreme weather events, rapidly occurring climate change, pollution, and new types of parasites.” She hopes this work will promote further research on maintaining species with large and diverse genetic structures. Westram concluded: “Perhaps this research helps convince people to protect many natural habitats so that species do not lose their genetic diversity.”
