Differences in rheotactic responses contribute to divergent habitat use between parapatric lake and stream threespine stickleback

Migration among populations is widely thought to undermine adaptive divergence, assuming gene flow arises from random movement of individuals. If individuals instead differ in dispersal behavior, phenotype-dependent dispersal can reduce the effective rate of gene flow or even facilitae divergence. For example, parapatric populations of lake and stream stickleback tend to actively avoid dispersing into the adjoining habitat.. However, the behavioral basis of this non-random dispersal was previously unknown. Here we show that lake and stream stickleback exhibit divergent rheotactic responses (behavioral response to currents). During the breeding season, wild-caught inlet stream stickleback were better than lake fish at maintaining position in currents, faced upstream more, and spent more time in low-current areas. As a result, stream fish expended significantly less energy in currents than did lake fish. These divergent rheotactic responses likely contribute to divergent habitat use by la...