Evolution in real time: Scientists predict—and witness—evolution in a 30-year marine snail experiment

Creationists who challenged us to show more evidence: this is for you!

Snails on a tiny rocky islet evolved before scientists' eyes. The marine snails were reintroduced after a toxic algal bloom wiped them out from the skerry. While the researchers intentionally brought in a distinct population of the same snail species, these evolved to strikingly resemble the population lost over 30 years prior.

The study is published in Science Advances.

It is 1988. The Koster archipelago, a group of islands off the Swedish west coast near the border with Norway, is hit by a particularly dense bloom of toxic algae, wiping out marine snail populations. But why would anyone care about the fate of a bunch of snails on a three-square-meter rock in the open sea? As it turns out, this event would open up the opportunity to predict and see evolution unfolding before our eyes.

Before, the islands and their small intertidal skerries—rocky islets—were home to dense and diverse populations of marine snails of the species Littorina saxatilis. While the snail populations of the larger islands—some of which were reduced to less than 1%—were restored within two to four years, several skerries could not seem to recover from this harsh blow.

Researchers saw a unique opportunity. In 1992, they re-introduced L. saxatilis snails to their lost skerry habitat—starting an experiment that would have far-reaching implications more than 30 years later.

L. saxatilis is a common species of marine snail found throughout the North Atlantic shores, where different populations evolved traits adapted to their environments. These traits include size, shell shape, shell color, and behavior.

The differences among these traits are particularly striking between the so-called Crab- and Wave-ecotype. These snails have evolved repeatedly in different locations, either in environments exposed to crab predation or on wave-exposed rocks away from crabs.

Wave snails are typically small, and have a thin shell with specific colors and patterns, a large and rounded aperture, and bold behavior. Crab snails, on the other hand, are strikingly larger, have thicker shells without patterns, and a smaller and more elongated aperture. Crab snails also behave more warily in their predator-dominated environment.

The Swedish Koster archipelago is home to these two different L. saxatilis snail types, often neighboring one another on the same island or only separated by a few hundred meters across the sea. Before the toxic algal bloom of 1988, Wave snails inhabited the skerries, while nearby shores were home to both Crab and Wave snails. This close spatial proximity would prove crucial.

Seeing that the Wave snail population of the skerries was entirely wiped out due to the toxic algae, researchers decided in 1992 to reintroduce snails to one of these skerries, but of the Crab-ecotype. With one to two generations each year, they rightfully expected the Crab snails to adapt to their new environment before scientists' eyes.

They then  saw evidence of the snails' adaptation already within the first decade of the experiment.

Over the experiment's 30 years, they were able to predict robustly what the snails will look like and which genetic regions will be implicated. The transformation was both rapid and dramatic.

However, the snails did not evolve these traits entirely from scratch. Some of the genetic diversity was already available in the starting Crab population but at low prevalence. This is because the species had experienced similar conditions in the recent past. The snails' access to a large gene pool drove this rapid evolution.

The team examined three aspects over the years of the experiment: the snails' phenotype, individual gene variabilities, and larger genetic changes affecting entire regions of the chromosomes called "chromosomal inversions."

In the first few generations, the researchers witnessed an interesting phenomenon called "phenotypic plasticity." Very soon after their transplantation, the snails modified their shape to adjust to their new environment. But the population also quickly started to change genetically. The researchers could predict the extent and direction of the genetic changes, especially for the chromosomal inversions.

They showed that the snails' rapid and dramatic transformation was possibly due to two complementary processes. A fast selection of traits already present at a low frequency in the transplanted Crab snail population and gene flow from neighboring Wave snails that could have simply rafted over 160 meters to reach the skerry.

In theory, scientists know that a species with high enough genetic variation can adapt more rapidly to change. However, few studies aimed to experiment with evolution over time in the wild.

This work allows scientists to have a closer look at repeated evolution and predict how a population could develop traits that have evolved separately in the past under similar conditions.

The team now wants to learn how species can adapt to modern environmental challenges such as pollution and climate change.

Not all species have access to large gene pools and evolving new traits from scratch is tediously slow. Adaptation is very complex and our planet is also facing complex changes with episodes of weather extremes, rapidly advancing climate change, pollution, and new parasites.

Finally happy news: Now, the snails researchers brought to the skerry in 1992 have reached a thriving population of around 1,000 individuals.

Crab-ecotype snails (1992) evolved to strikingly resemble the lost Wave-ecotype snails on a skerry. Credit: ISTA, images by Kerstin Johannesson

Diego Garcia Castillo et al, Predicting rapid adaptation in time from adaptation in space: A 30-year field experiment in marine snails, Science Advances (2024). DOI: 10.1126/sciadv.adp2102. www.science.org/doi/10.1126/sciadv.adp2102