Island or just island‐like? It makes a big difference to pike population structure

Islands have long played an important role in our understanding of evolutionary processes. Perhaps the most famous islands in this context are the Galápagos Islands. Visited by Charles Darwin in 1835 while on the HMS Beagle, observations there, including mockingbirds, tortoises and finches, played a pivotal role in developing the theory of evolution by natural selection (Shivanna, 2018). Islands have unique properties that make them valuable to evolutionary biologists, including distinct boundaries, geographical isolation, relatively recent formation (on geological timescales), simplified biotas and island chains that can provide replication (Losos & Ricklefs, 2009). While islands are normally thought of through the terrestrial lens, key principles also apply to the surrounding reef habitats, with fish assemblages reflecting patterns predicted by island biogeography theory (Nogueira et al., 2026). The island concept can be taken one step further, with isolated bodies of water like lakes acting as islands for the fully aquatic organisms that live in them. Categorised as island-like systems (Itescu, 2019), lakes hold many of the same characteristics which make true islands so valuable for studying evolution. As adaptive radiations go, Darwin's finches (18 species) are left looking quite underwhelming when compared to the 500–800 endemic cichlid species in Lake Malawi (Joyce et al., 2011). Indeed, cichlids in crater lakes have also given us some of the best examples of sympatric evolution in action (Malinsky et al., 2015). In this issue, Diaz-Suarez et al. (2026) take island thinking to the next level by exploring evolutionary forces acting on northern pike (Esox lucius L.) populations from both an island and an inverse island (a lake). The beauty of the study is the paired experimental design, using samples from Lake Vänern (Sweden) [Europe's third largest lake (Berg, 1985)] and the brackish areas around the island of Saaremaa (Estonia), which occupy similar spatial scales. Indeed, the scales are such that it looks as if someone has taken a cookie cutter to Sweden, leaving the lake behind, and then placed that land in the Baltic Sea to make the island of Saaremaa. Diaz-Suarez et al. (2026) combine pike samples taken from these wonderfully complementary formations with sophisticated genomic techniques, generating 3641 million raw reads and detecting 5778 SNPs to assess evolutionary forces at play. What the results show is that, despite similar spatial scales, the lake showed higher genetic diversity and extremely low genetic structuring, while the samples from the brackish areas around the island showed strong genetic structuring and lower genetic diversity. Additionally, in the island samples, genetic differentiation significantly increased with the distance between sample sites, a trend which was not seen in the lake. In the lake, samples seperated by 200 km showed no greater genetic differentiation than those just a few kilometres from each other. Authors suggest that the reason for this is that in the lake, highly mobile large pelagic pike are common, whereas in coastal areas around the island, this is not the case, potentially due to high mortality from predators like the Baltic grey seal (Halichoerus grypus). Other hypotheses such as more continuous and abundant spawning habitats in freshwater, rather than that found with brackish habitats, may also play a role. While genomic structuring in the Baltic Sea has been explored previously, the insights from Diaz-Suarez et al. (2026) provide a valuable contrast with a large lake system, where genomic structuring is not well explored. On a practical level, it means that pike in Lake Vänern can be managed as a single stock. But these results also show that not all types of island elicit the same evolutionary forces to the aquatic organisms that live in or around them.