Data from: 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. Models of migration-selection balance typically assume that gene flow arises from random diffusion of individuals across a landscape. If individuals instead differ in dispersal behavior or habitat preferences, then movement ceases to be random, the effective rate of gene flow may be lower, and dispersal may even promote adaptive divergence. For example, previous work has found that 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 test whether lake and stream stickleback exhibit divergent rheotactic responses (behavioral response to currents). We found that during the breeding season, wild-caught inlet stream stickleback were better than lake fish at maintaining position in currents, spent a larger percentage of time facing towards currents, and spent more time in low-current boundary areas. As a result, lake fish expended significantly more energy in currents than did stream fish. These divergent rheotactic responses likely contribute to divergent habitat use by lake and stream stickleback. Although rheotactic differences were absent in non-breeding wild or lab-reared fish, divergent behavior of breeding-season fish may suffice to facilitate assortative mating by breeding location. The resulting reproductive isolation between lake and stream fish may explain our observation of evolutionary differentiation across remarkably fine spatial scales in parapatric stickleback populations.