Genes with major phenotypic effects facilitate quantifying the contribution of genetic vs. plastic effects to adaptive divergence. A classical example is Ectodysplasin (Eda), the major gene controlling lateral plate phenotype in three-spined stickleback. Completely plated marine stickleback populations evolved repeatedly towards low-plated freshwater populations, representing a prime example of parallel evolution by natural selection. However, many populations remain polymorphic for lateral plate number. Possible explanations for this polymorphism include relaxation of selection, disruptive selection, or...