A small rocky island lost at sea has become the stage for an evolutionary transformation. Marine snails that were reintroduced evolved, in just a few decades, to resemble those that had disappeared thirty years earlier. This unique experiment reveals the incredible ability of species to rapidly adapt to their environment.
It all started in 1988, when the coasts of the Koster Archipelago in Sweden were struck by a toxic algal bloom. The event decimated the population of marine snails of the species Littorina saxatilis on several islets. An ecological catastrophe that would, however, pave the way for a unique experiment.
The crab ecotype (left) is larger and more cautious. The wave ecotype (right) is smaller and bolder.
Credit: David Carmelet
Four years after this disaster, marine biologist Kerstin Johannesson from the University of Gothenburg decided to reintroduce these mollusks on one of the affected small islands. However, the snails she chose belonged to a distinct variety called the "crab type," not the ones that originally populated the area, known as the "wave type."
These two types of snails differ in both their morphology and behavior. The crab type, larger with a thick shell, is adapted to environments where predators abound. The wave type, smaller and bolder, thrives on rocks battered by waves, far from major threats.
The key element of this experiment is the geographical isolation of the islands. On the rocks where the wave-type snails had once prospered, introducing crab-type snails in 1992 allowed researchers to observe how a population can quickly adapt to a different environment.
By the first decade, researchers had already observed adaptations in the reintroduced population. The snails began to change shape to better adapt to the waves characteristic of their new environment. An evolutionary process accelerated by the genetic richness of this species, according to Anja Marie Westram, co-author of the study.
Crab-type snails (1992) evolved to resemble the now-extinct "wave type" snails.
Credit: ISTA, images by Kerstin Johannesson
The snails did not start from scratch in developing their new traits. Genetic variations, which were rare in the original population, were selected and amplified, allowing this rapid transformation. Genes from neighboring populations may have also influenced this evolution.
The experiment allowed the study of both the phenotypic and genetic changes in the snails. Notably, chromosomal inversions—segments of genes involved in specific adaptations—were identified, making evolution even quicker. This study helps scientists better understand how a population can develop traits already observed under similar conditions.
The results of this study are crucial at a time when many species are forced to adapt to rapid climate fluctuations and pollution. Researchers hope this will encourage the preservation of natural habitats rich in genetic diversity, which is essential for species survival.
What is evolution in real time?
Evolution in real time refers to the direct observation of evolutionary changes in a population over a relatively short period, typically a few decades or less. This contrasts with the traditional view of evolution, often perceived as a process occurring over millions of years.
In the case of the marine snails Littorina saxatilis, introduced to a small island after a natural disaster, evolution was observed over a span of just thirty years. They altered their appearance and behavior to adapt to a new environment. This rapid ability to evolve highlights the genetic and phenotypic plasticity of certain species.
The rapid adaptation of the marine snails is due to two main factors: the natural selection of traits already present at low frequency in the original population and genetic exchange with neighboring populations. These combined processes allowed for accelerated evolution, proving that some species can quickly respond to environmental changes when they have sufficient genetic diversity.
This concept of rapid evolution is crucial in a world where climate and ecosystems are changing at an accelerating pace. It could help researchers understand how species respond to increasing environmental pressures, such as global warming and pollution.