Before revision:  Using this model, we show that biased mutation rates and directional selection alone can cause  polymorphism rates within and substitution rates between populations that are  ascribed to balancing or overdominant selection by the McDonald-Kreitman test 26 .
After revision:  Using this model, we show that biased mutation rates and directional selection alone can cause patterns of polymorphism rates within and substitution rates between populations that are ascribed to balancing or overdominant selection by the McDonald-Kreitman test 26 .