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A simple change in species composition can impact the course of evolution: A research team from the University of Bern and the University of British Columbia in Canada shows that the presence of just one other fish species is enough to drive the emergence of new species in sticklebacks.

It has long been assumed that adaptation to different habitats plays an important role in the evolution of new species. Yet how important this influence truly is—particularly during the initial stages of the speciation process—and which ecological differences are most critical remain major questions in evolutionary research.

For the current study, the research team studied populations of threespine stickleback—small fish about the size of a finger—from lakes in western Canada. These lakes formed after glaciers from the last ice age melted less than 12,000 years ago and were then colonized by sticklebacks from the sea. While many of these lakes are environmentally similar, they differ in one aspect: in some, another fish species, the prickly sculpin, lives alongside sticklebacks, while in other lakes sculpins are absent.

This seemingly simple ecological difference—living with or without sculpins—has repeatedly pushed sticklebacks down distinct evolutionary paths: in lakes with sculpins, sticklebacks have evolved into slimmer open-water forms, while in sculpin-free lakes they have become stockier bottom-feeding specialists.

"Our study sheds light on how new species form and on the role ecological differences play in initiating this process," says Dr. Marius Roesti from the Institute of Ecology and Evolution at the University of Bern. Roesti is the first author of the study, which has just been in the Proceedings of the National Academy of Sciences. He initiated the research project during his postdoctoral work at the University of British Columbia in Canada and completed it at the University of Bern.

Mating isolation as a decisive step on the way to new species

To test whether the ecological differences between sticklebacks from these different lake types have triggered the evolution of new species, the researchers caught several hundred sticklebacks from multiple lakes with and without sculpins and introduced them into large experimental ponds. A genetic parentage analysis of more than 400 offspring revealed that the fish tended to mate with partners from the same lake type.

In some cases, this mate preference was so strong that populations from different lake types separated completely. Roesti explains, "Being able to witness the emergence of new species was only possible through our experimental approach. By bringing these evolutionarily young populations from distinct lakes together in the same ponds, we could directly test whether they remained reproductively isolated—and indeed, sticklebacks from different lake types partially did so despite ample mating opportunities—a decisive step on the path toward becoming new species."

Interaction with a single, ecologically similar species crucial for evolution

The study further shows that the degree of isolation was not random but depended on how much populations had adapted, in both body shape and the genome, to the presence of sculpins. Put simply, the more populations differed in shape and genes between lakes with and without sculpins, the less willing they were to mate.

"Remarkably, sculpins and sticklebacks hardly interact directly. Instead, they compete for similar prey and share some of the same predators, influencing each other mainly indirectly through these ecological links," explains Roesti.

Close, direct relationships between species, such as between hosts and parasites or plants and pollinators, have long been seen as powerful engines of speciation. "Our results suggest that indirect interactions between ecologically similar species can also ignite this evolutionary process," says Roesti.

More broadly, the results show that adaptation to different ecological conditions can, even between geographically isolated populations, be an important driver of speciation. Although geographic isolation is often viewed as a prerequisite for speciation to begin, the importance of ecological adaptation at these early stages remains controversial.

"Our study shows that even in geographically isolated populations, speciation does not necessarily unfold slowly or by chance—adaptation to subtle ecological differences can be enough to trigger the process in surprisingly little time," says Roesti.

Delicate evolutionary balance between interacting species

The study makes clear that even small ecological differences have the potential to shape the course of evolution. It also highlights how sensitive interactions between species are for biodiversity as a whole: depending on which and how such interactions change, new species can form or existing ones can disappear.

"We recently learned that one of the stickleback populations we studied has gone extinct after a predatory, non-native fish species was introduced into its lake. This is a sad example showing how human interventions can alter species coexistence and thereby profoundly influence natural evolutionary processes," says Roesti.

The researchers are continuing their investigations into the formation of species—including in waters in Central Europe, such as in the Lake Constance region, where sticklebacks have also adapted to different habitats.

A continuing focus remains the use of field experiments to better understand the mechanisms of speciation under natural conditions. By combining such experiments with high-resolution laboratory analyses, the researchers hope to gain further insights into the evolutionary mechanisms underlying biological diversity.