We have studied the effect of molecular beam epitaxy growth conditions on the surface morphology of strained InAs/GaAs(331)A films. Our results reveal that InAs nanowires aligned along the [110] direction are formed under As-rich conditions, which is explained by the effect of anisotropic buffer layer surface roughing. Under In-rich conditions, however, the surface morphology of the InAs layers is characterized by a feature of island-pit pairs. In this case, cooperative nucleation of islands and pits can lower the activation barrier for domain growth. These results suggest that the surface morphology of strained InAs layers is highly controllable.