Brain evolution is a hot topic nowadays (and was in the last several decades), perhaps because we humans are so proud of our relative brain size. The vast majority of studies so far was conducted on an interspecific basis, and revealed a number of interesting (phylogenetically controlled) correlations from a positive correlation between relative telencephalon size and habitat complexity to hypocampus volume and food hoarding (there are even more surprising correlations out there, like negative correlation between reative brain size and relative testis size).
On the other hand, intraspecific studies are extremely scarce, while potentially interesting: one can find the populations (most probably) in the habitats that posed the different selective pressures, or, with proper data, the roles of selection and drift can be directly tested. Here, we compared first-generation lab-bred nine-spined sticklebacks (Pungitius pungitius) from strikingly different habitats. We used four populations, two marine (Baltic vs. White Seas) and two small isolated pond populations (separated by > 500km). Marine populations are members of a diverse fish community with many predators and competitors, while in ponds the stickleback is the only fish. While it’s almost impossible to quantify the habitat complexity of marine nine-spined sticklebacks (yearly habitat use and potential migrations are unknown), the ponds (surface area < 5ha) were overly simple.
We found significant but habitat-independent population differences in relative brain size and cerebellum size in five months old fish (they have approached adult size). More interestingly, we found habitat-specific (and independent from population origin) differences in relative telencephalon and bulbus olfactorius size: marine fish had larger structures. The repeated, habitat-specific pattern suggests natural selection to be responsible for the pattern. Seemingly, the lower (biotic and abiotic) habitat diversity in ponds relaxed the selection on neural processes related to telencephalon and bulbus olfactorius, resulting in the reduction of these energetically costly organs. We hope that our results will provoke more research in this -so far neglected- topic (i.e. intraspecific brain evolution), leading to a deeper understanding of brain evolution.
Ref.: Gonda A, Herczeg G, Merilä J 2009. Adaptive brain size divergence in nine-spined sticklebacks (Pungitius pungitius)? Journal of Evolutionary Biology, in press.