Collective behavior diverges independently of the benthic-limnetic axis 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.