Population Dynamics of the Segregation Distorter Polymorphism of DROSOPHILA MELANOGASTER.

Two two-locus models of the population dynamics of the segregation distortion (SD) polymorphism of Drosophila melanogaster are described. One model is appropriate for understanding the population genetics of SD in nature, whereas the other is a special case appropriate for understanding an artificial population that has been extensively analysed. The models incorporate the general features of the Sd and Rsp loci which form the core of the SD system. It is shown that the SD polymorphism can be established only when there is sufficiently tight linkage between Sd and Rsp. An approximate treatment, valid for tight linkage, is given of all the equilibria of the system and their stabilities. It is shown that the observed composition of natural and artificial populations with respect to the Sd and Rsp loci is predicted well by the model, provided that restrictions are imposed on the fertilities of certain genotypes. Highly oscillatory paths towards equilibrium are usually to be expected on the basis of this model. The selection pressures on inversions introduced into this system are also investigated.