Ecological speciation in sticklebacks: Environment-dependent hybrid fitness

"Ecological" speciation occurs when reproductive isolation evolves as a con sequence of divergent selection between populations exploiting different re sources or environments. We tested this hypothesis of speciation in a young stickleback species pair by measuring the direct contribution of ecologica l selection pressures to hybrid fitness. The two species (Limnetic and bent hic) are strongly differentiated morphologically and ecologically, whereas hybrids are intermediate. Fitness of hybrids is high in the laboratory, esp ecially Fl and Fz hybrids(backcrosses may show some breakdown). We transpla nted FI hybrids to enclosures in the two main habitats in the wild to test whether the distribution of resources available in the environment generate s a hybrid disadvantage not detectable in the laboratory. Hybrids grew more slowly than limnetics in the open water habitat and more slowly than benth ics in the littoral zone. Growth of Fl hybrids was inferior to the average of the parent species across both habitats, albeit not significantly. The c ontrast between laboratory and field results supports the hypothesis that m echanisms of Fl hybrid fitness in the wild are primarily ecological and do not result from intrinsic genetic incompatibilities. Direct selection on hy brids contributes to the maintenance of sympatric stickleback species and m ay have played an important role in their origin.