Does plasticity enhance or dampen phenotypic parallelism? A test with three lake-stream stickleback pairs.

Parallel (and convergent) phenotypic variation is most often studied in the wild, where it is difficult to disentangle genetic versus environmentally-induced effects. As a result, the potential contributions of phenotypic plasticity to parallelism (and non-parallelism) are rarely evaluated in a formal sense. Phenotypic parallelism could be enhanced by plasticity that causes stronger parallelism across populations in the wild than would be expected from genetic differences alone. Phenotypic parallelism could be dampened if site-specific plasticity induced differences between otherwise genetically-parallel populations. We used a common-garden study of three independent lake-stream stickleback population pairs to evaluate the extent to which adaptive divergence has a genetic or plastic basis, and to investigate the enhancing versus dampening effects of plasticity on phenotypic parallelism. We found that lake-stream differences in most traits had a genetic basis, but that several traits als...