Multi-locus self-incompatibility systems offer few obvious adaptive advantages to the species possessing them. However, the gametophytic system's independent gene action allows the separate genes in a two gene system to behave as if they were individually not involved in a systematic disruption of panmixia. Under such circumstances, fixation of one of the two genes is readily obtained if an allele possesses a selective advantage. The resulting single gene system (the classic Nicotiana system) is then resistant to disruption, except by genes which allow selfing, which rapidly reach fixation.