Evolutionary home advantage: Study reveals adaptation to be surprisingly local

Since Charles Darwin, natural selection has been recognized as the major driving force of evolution. Genetically heritable traits that give individuals an advantage in their local environment increase in frequency over generations. As a result, populations become evolutionarily adapted to their environments and can gradually diverge from one another – ultimately giving rise to distinct new species. Recurring patterns in nature, such as similar body shapes in similar habitats, are therefore often taken as evidence for natural selection: individuals with certain traits had an advantage in their particular environment and were consequently more likely to survive and reproduce. However, demonstrating that a population is truly adapted to its local environment requires a more direct test: individuals must perform better in their own habitat than individuals originating from elsewhere.

This is where a new study led by Dr. Marius Roesti of the Institute of Ecology and Evolution at the University of Bern comes in. Through a carefully designed field experiment, the researchers were able not only to test evolutionary adaptation directly, but also to determine how specifically populations are adapted to local conditions. Their model organism was the threespine stickleback (Gasterosteus aculeatus), a small fish found in a wide range of habitats across the Northern Hemisphere – from large lakes to small streams – where populations repeatedly evolve similar characteristics under similar conditions. As the new experimental results reveal, however, differences among stickleback populations do not simply reflect adaptation to broad habitat types such as “lake” and “stream.” Instead, much of their adaptation goes beyond these broad habitat differences and is finely tuned to the unique conditions of individual locations. The findings shed new light on the fine scale at which biodiversity arises and have important implications for its conservation. The study was published in the Proceedings of the National Academy of Sciences (PNAS).

Large-scale field experiment traces evolution in action

For their field experiment, the research team bred sticklebacks originating from Lake Constance – one of the largest lakes in mainland Europe – and from four tributary streams in Switzerland, Liechtenstein, and Germany. The fish were raised for two generations under identical laboratory conditions at the University of Bern. By the time they were released into the wild, they had therefore never experienced the natural habitats in which their grandparents had lived. The researchers placed more than 700 of these fish into specially designed enclosures at three natural field sites: in Lake Constance itself and in two of the streams from which the original populations had been sampled, one in Switzerland and one in Liechtenstein. This experimental design allowed the researchers to compare how well fish from different populations survived and grew under different natural conditions.

Every stream is unique – and so are its fish

Previous studies had found recurring differences between lake and stream stickleback populations in nature. Consistent with these observations, the new experiment showed that lake fish outperformed stream fish in the lake, while stream fish performed better in streams – clear evidence of adaptation to the contrasting “lake” and “stream” environments. Surprisingly, however, much of the adaptation uncovered by the researchers went beyond this broad habitat contrast. Instead, the experiment revealed adaptation at a much finer scale: at both stream sites, fish originating from that particular stream performed best. Local stream fish therefore clearly outperformed not only lake fish, but also fish from other streams – some originating only a few kilometers away. “Because all the fish had been raised under the same laboratory conditions for two generations, we can rule out individual experience or direct environmental effects during their lifetime as explanations for these differences. What we see instead must reflect heritable differences that evolved through natural selection over many generations,” says Roesti. “To us, two small streams may look almost identical. But to the fish, they obviously represent very different worlds. A stream is not simply a stream.”

Populations are not interchangeable

The study emphasizes that biodiversity extends far beyond differences between species. “Our results show that natural selection can make populations of the same species remarkably unique – despite living in habitats that may appear very similar to us,” says Roesti. Local populations can therefore harbor genetic adaptations that have been favored by natural selection over many generations, making them an important component of biological diversity. “When we think of biodiversity, we often think of the diversity of species,” says Roesti. “Our study shows that evolutionary differences within a species also represent an important part of biodiversity.” The findings therefore have direct implications for conservation. “Populations are not freely interchangeable. If animals or plants are moved from one place to another, they may quickly find themselves in conditions to which they are not so well adapted – and therefore struggle to survive or reproduce. Local adaptation should thus receive greater consideration in conservation, particularly in reintroduction and relocation projects,” explains Roesti.

How experiments reveal hidden adaptations

Much of modern biology relies on detecting patterns in large datasets, abstract computer models, and complex statistical analyses – all increasingly supported by artificial intelligence. “Such modern approaches can reveal important insights. But ultimately, whether a scientific explanation really holds can only be tested experimentally,” says Roesti. Experiments conducted under real-world conditions are particularly powerful. The new study provides a compelling example: it shows how previously hidden local adaptation can be revealed directly through experiments conducted in nature. At a time when science is becoming increasingly technology-driven, Roesti believes that such direct “reality checks” are particularly important. And in experimental field research, humans remain indispensable. “Field experiments require us not only to think carefully, but also to find practical ways of turning ideas into experiments that actually work in nature. So far, only humans can meet these complementary demands.” Applying similar experimental approaches to other organisms should reveal how broadly the findings extend across nature. “So there is still plenty of work to be done,” Roesti concludes.