Data from: Independent axes of genetic variation and parallel evolutionary divergence of opercle bone shape in threespine stickleback

Evolution of similar phenotypes in independent populations is often taken as evidence of adaptation to the same fitness optimum. However, the genetic architecture of traits might cause evolution to proceed more often toward particular phenotypes, and less often toward others, independently of the adaptive value of the traits. Freshwater populations of Alaskan threespine stickleback have repeatedly evolved the same distinctive opercle shape after divergence from an oceanic ancestor. Here we demonstrate that this pattern of parallel evolution is widespread, distinguishing oceanic and freshwater populations across the Pacific Coast of North America and Iceland. We test whether this parallel evolution reflects genetic bias by estimating the additive genetic variance-covariance matrix (G) of opercle shape in an Alaskan oceanic (putative ancestral) population. We find significant additive genetic variance for opercle shape and that G has the potential to be biasing, because of the existence of regions of phenotypic space with low additive genetic variation. However, evolution did not occur along major eigenvectors of G, rather occurred repeatedly in the same directions of high evolvability. We conclude that the parallel opercle evolution is most likely due to selection during adaptation to freshwater habitats, rather than due to biasing effects of opercle genetic architecture.独立种群中相似表型的演化，常被视作适应同一适合度最优值的佐证。然而，性状的遗传架构可能会令演化更倾向于朝向特定表型进行，而非其他表型，且这一过程不受性状适应性价值的影响。阿拉斯加三棘刺鱼的淡水种群在由海洋祖先类群分化而来后，反复演化出了相同的独特鳃盖骨形状。本研究证实，该平行演化模式广泛存在，可区分北美太平洋沿岸与冰岛的海洋及淡水种群。我们通过估算阿拉斯加海洋种群（推定的祖先类群）鳃盖骨形状的加性遗传方差-协方差矩阵（additive genetic variance-covariance matrix，G），检验该平行演化是否由遗传偏差所驱动。研究发现，鳃盖骨形状存在显著的加性遗传方差；且由于表型空间中存在加性遗传变异水平较低的区域，G具备引发演化偏差的潜力。但实际演化并未沿着G的主要特征向量进行，而是反复朝着高可演化性的同一方向发生。综上，鳃盖骨的平行演化更可能是适应淡水生境过程中选择作用的结果，而非鳃盖骨遗传架构的偏差效应所致。