The Persistence of Abnormal Sex Ratios in the African Butterfly, Acraea Encedon

The African butterfly, Acraea encedon, contains a directly inherited, all-female strain. Elementary population genetics predicts that this strain should lead to the eventual extinction of the species, but in fact the strain appears to have reached equilibrium. 2. Several hypotheses have been put forward to explain the maintenance of the sex ratio polymorphism in this species, but so far none has appeared convincing. 3. Two previously suggested hypotheses, one involving a selective advantage for the normal females by virtue of their time of emergence and the other involving successive extinction and recolonization of discrete populations, are combined. It is shown by means of a simulation that these two hypotheses together lead to a greater degree of stability than either separately.