The scientists examined stickleback populations inhabiting lakes formed less than 12,000 years ago after glaciers retreated. While many lakes share similar environmental conditions, they differ in one key factor: some host both sticklebacks and prickly sculpins, while others lack sculpins entirely. This simple difference has repeatedly driven sticklebacks down separate evolutionary paths. In lakes with sculpins, sticklebacks have evolved into slender, open-water types; in sculpin-free lakes, they have developed into robust bottom feeders.
"Our study sheds light on how new species form and on the role ecological differences play in initiating this process," said Dr. Marius Roesti from the Institute of Ecology and Evolution at the University of Bern. Roesti initiated the work while at the University of British Columbia, supported by the Swiss National Science Foundation, and completed it in Bern.
To determine whether these ecological distinctions influenced reproductive isolation, the team conducted experiments in large outdoor ponds. They introduced several hundred sticklebacks from multiple lake types, later analyzing more than 400 offspring through genetic parentage testing. The results revealed that fish tended to mate with others from the same lake type, sometimes exclusively, demonstrating a strong preference that could lead to reproductive isolation. "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," Roesti explained.
The study found that the degree of isolation correlated with how much the populations had adapted - both morphologically and genetically - to the presence or absence of sculpins. Remarkably, the two fish species rarely interact directly; instead, they compete for similar prey and share predators, influencing one another indirectly. "Our results suggest that indirect interactions between ecologically similar species can also ignite this evolutionary process," said Roesti.
The findings demonstrate that adaptation to subtle ecological differences can drive speciation even among geographically isolated populations. While isolation has often been viewed as essential for new species to arise, this research shows that ecological pressures alone can rapidly initiate the process.
The work also underscores how sensitive ecological relationships are to human influence. Roesti noted that one of the studied stickleback populations recently went extinct following the introduction of a predatory, non-native fish, highlighting how changes in species coexistence can disrupt natural evolution.
The team plans to continue exploring the mechanisms of speciation in other regions, including Central Europe's Lake Constance system, using a combination of field experiments and genomic analyses to better understand the origins of biodiversity.
Research Report:A species interaction kick-starts ecological speciation in allopatry
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