Adaptive divergence and the evolution of hybrid trait mismatch in threespine stickleback

Hybrid fitness can be negatively impacted by trait mismatch, whereby hybrids resemble one parent population for some phenotypic traits and the other parent population for other traits. In this study, we used threespine stickleback fish (Gasterosteus aculeatus L.) to test whether trait mismatch in hybrids increases with the magnitude of phenotypic divergence between parent populations. We measured morphological traits in parents and hybrids in crosses between a marine population representing the ancestral form and twelve freshwater populations that have diverged from this ancestral state to varying degrees according to their environments. We found that trait mismatch was greater in more divergent crosses for both F1 and F2 hybrids. In the F1, the divergence-mismatch relationship was caused by traits having dominance in different directions whereas it was caused by segregating phenotypic variation in the F2. Selection against mismatched traits is an ecological analogue to selection against intrinsic hybrid incompatibilities, and our results imply that extrinsic hybrid incompatibilities accumulate predictably as phenotypic divergence proceeds.