Discrimination under imperfect competition - a way of earning or wasting money?

How would firms allocate their employees between two differently paid jobs? Suppose that a number of equally skilled workers belonging to two distinguishable groups are competent enough for both positions. It is then profitable to employ the group with the higher labour supply elasticity in the high wage occupation. This explains why less wealthy groups are discriminated against, but it suggests that discrimination of women may be a waste from the standpoint of the employer in an important category of cases.