Scientists often don’t see evolution in action. So they were very excited when they saw a few snails on a small rocky island in Sweden evolving into a completely different population that had disappeared thirty years earlier. The change was amazing.
In 1988, the Koster Archipelago, a group of islands off the western coast of Sweden, experienced a toxic algae bloom. The sea snails that lived on a 3-square-meter rock in the archipelago were eliminated. It’s not something very serious in itself, these things happen. But it provided the scientists with an interesting research topic: They introduced a new population of the same type of snail into the rock and watched what happened. This was more extreme than they expected. In a short time, the snails evolved in a way that became strikingly similar to groups that had been lost three decades earlier. So they saw evolution happening before their eyes.
A special experience
Before the arrival of toxic algae, the islands and their small rocks were home to large and diverse populations of sea snails of this type Litorina saxatilis. But while the snail populations that were also wiped out on the larger islands recovered within two to four years, the snail populations on the smaller rocky islands did not.
So marine ecologist Kirsten Johansson from the University of Gothenburg saw a unique opportunity. In 1992 she reintroduced the snail L. saxatilis to its ancient habitat on the rocks. Thus began an experiment that would have far-reaching consequences more than thirty years later.
Crab and wave snails
L. saxatilis is a common marine snail along the North Atlantic coast, where several populations have evolved traits adapted to their environment. Consider their size, shell shape, shell color and behavior. The differences are even greater between the so-called crabs and wave snails. These animals have evolved in places where they are threatened by crabs or where waves threaten to wash them away, and they are not crabs.
Therefore, Gulf snails are usually small in size and have a thin shell with specific colors and patterns, and a large, round opening, and they crawl without fear. Crab snails, on the other hand, are much larger, have thicker shells without patterns and a smaller, more elongated opening. Crab snails also behave more cautiously, because they are afraid of crabs.
Critical neighbors
The Swedish Köster Archipelago is home to two different species of L. saxatilis snails, which often coexist on the same island or are only separated by a few hundred meters of sea. Before the toxic algae bloom in 1988, bay snails inhabited the rocks, while marine and bay snails lived nearby. The latter will be crucial. When Johansson saw that the population of Gulf snails on the rocks had been completely wiped out by toxic algae, she decided in 1992 to return the snails to one of these rocks, but this time it was a crab. With one or two generations per year, I rightly expected the crab snails to adapt to their new environment.
Fast and intense
“Our colleagues saw evidence of snail adaptation during the first decade of the experiment,” says researcher Diego Garcia Castillo of the University of California, California. Institute of Science and Technology Austria (ISTA). “Over thirty years of experimentation, we were able to clearly predict what the snails and the genes involved would look like. “The transformation was fast and furious,” he adds.
However, snails did not completely develop these characteristics from scratch. Researcher Anja Marie Westram from Norway Nord University “Some genetic diversity was already present in the original crab snail population, but at a low rate of spread,” he explains. “This is because the species experienced similar conditions in the recent past. The snails’ access to a large pool of genes stimulated rapid evolution.”
Floating across the sea
In the first few generations after moving to rock, the snails actually changed their shape to adapt to their new environment. But the population also began to change rapidly genetically. The researchers were able to predict the magnitude and direction of genetic changes.
They showed that the rapid transformation of snails may have been due to two complementary processes: rapid selection for traits that were already present to a limited extent in the translocated crab snail populations, and the exchange of genes with nearby bay snails that could easily float 160 metres. The sea to reach the rock.
Climate change
In theory, scientists know that species with enough genetic diversity can adapt to changes more quickly, but few studies have tracked this in the wild. “This work allows us to take a closer look at evolution and predict how a population might develop traits that evolved separately in the past under similar conditions,” Garcia-Castillo says.
The team now wants to learn how species can adapt to pollution and climate change, for example. “Not all species have access to large gene pools, and developing new traits from scratch is a very slow process. Adaptation is very complex, and our planet is also facing extreme weather conditions, climate change, pollution, and new parasites,” Westram concludes. “This research helps convince people to protect certain natural habitats so that species do not lose their genetic diversity.”
Meanwhile, the snails on the rock are doing very well. The population that Johansson brought to the island in 1992 has grown to nearly a thousand healthy individuals.
