Sexual conflict
The same set of alleles in males and females may have different optima: they are expressed differently in the sexes. A classic example is the human pelvis, where females need larger hips for childbirth. A narrower hip size is better for locomotion. The genes that affect hip size must reach a compromise that is at neither the male optimum nor the female optimum. In some cases, the loci involved are expressed differently in males and females. Evidence indicates that intralocus conflict is important in the evolution of many traits.