Researchers proposed a way to watch evolution unfold and then tested the idea with a long-term study of sea snails. The goal was to observe how a population responds to a harsh environmental challenge and to track the tempo of natural selection as events unfold. Over thirty years, the team watched the process in the field, gathering observations that bridged theory and real life. The work offered a rare demonstration of evolution in action and helped anchor a new understanding of how quickly traits can arise when fitness landscapes shift.
During the late 1980s, a toxic algal bloom swept through the Koster archipelago off Sweden’s west coast, nearly wiping out Littorina saxatilis on the small, exposed islands. On the larger landmasses, populations recovered within a couple of years to levels close to normal, but the smallest islands suffered deep declines that were slow to reverse, if they recovered at all. This stark contrast created a natural experiment in resilience and adaptation across a shared geographic setting.
Seeing the disaster as a rare chance to study evolution at work, marine ecologist Kerstin Johannesson began what would become a thirty-year project by moving snails back to one island and letting a team of international researchers monitor the outcomes. They pursued a set of predictions about how the snails would change in body plan, how their behavior might shift in response to the new surroundings, and which parts of the genome would bear the signature of selection.
Shortly after settling, the snails began to exhibit changes in form that improved their fit to the island environment. Simultaneously, genetic analyses revealed rapid shifts in allele frequencies, signaling that evolution was not waiting for generations to pass but was happening in steps across decades and even within a single generation in some cases. The combined shift in phenotype and genotype underscored the tight link between an organism’s outward form and its genetic underpinnings.
Predictive patterns emerged as the study progressed. The snail populations regained numbers into the thousands, aided by individuals from other islands that carried preexisting traits. These migrants mixed with local populations, spreading advantageous combinations faster than would have happened through isolated drift alone. As new lineages fused and recombined, the archipelago’s snails collectively moved toward a suite of traits that improved survival under the island conditions, illustrating how migration fuels adaptation.
The results highlighted a central lesson about evolution: genetic diversity matters. When a population has access to a broad gene pool via mating with well-adapted partners, new traits can appear more quickly and spread more reliably. The study also demonstrated the power of gene flow across habitats to reshape whole communities, a reminder that connectivity among populations can accelerate adaptation in the face of change.
Earlier scientists noted that isolation and environmental stress have long been linked to the decline of large mammals such as mammoths, reminding researchers that demographic forces matter as much as selective forces. The snail study adds another piece to that story, showing how connected populations can weather shocks and evolve in real time rather than drifting toward extinction.