Collective behavior evolves independently of benthic-limnetic divergence in stickleback

Comparing populations across replicate environments or habitat types can help us understand the role of ecology in evolutionary processes. If similar phenotypes are favored in similar environments, parallel evolution may occur. Collective behavior, including collective movement (e.g., schooling, flocking) and social networks, can play a key role in the adaptation by animals to different environments. However, studies exploring the parallelism of collective behavior are limited, with research traditionally focusing on morphological traits. Here, we asked if collective behavior has evolved in parallel across replicate populations of benthic and limnetic three-spined stickleback (Gasterosteus aculeatus). There were repeatable, population-level differences in collective behavior in a common garden, with some populations forming groups that were more cohesive and with higher strength and clustering coefficients. This suggests that collective behavior can evolve. However, these differences were not predicted by ecotype (benthic vs. limnetic). We found no evidence that boldness or morphological traits – both of which are known to be associated with benthic-limnetic divergence – were correlated with collective behavior. Together, these results suggest that while collective behavior evolves in this system, it does not co-evolve with divergence along the benthic-limnetic axis.