If genetics played a roll in intelligence don't you think we'd see some intellectual differences between groups of genetically similar people(i.e. races), like we do with physical characteristics? Some races of people are taller than others on average, and some races are more resistant to certain diseases. Obviously we don't see differences like this when it comes to cognitive ability though.
Different ethnicities have different physical characteristics because they are adapted to different environments. Being able to efficiently use sunlight is useful in high altitudes, being able to tolerate heat and intensive sunlight is useful near the equator. Intelligence is useful everywhere.
