REINFORCEMENT: A NEW PERSPECTIVE ON AN OLD CONTROVERSY.

A critical component of the reinforcement model, incomplete postzygotic reproductive isolation, has been overlooked in the many selection experiments designed to test this model. Many field observations are consistent with the reinforcement model, but theoretical work predicts that reinforcement should only work when nearly complete postzygotic reproductive isolation has developed. The experiments described here used four levels of postzygotic reproductive isolation: complete, strong, moderate, and none. A significant decrease in heterotypic matings occurred over 25 generations in the complete postzygotic reproductive isolation treatment. No evidence for reinforcement was found in any of the other treatments using incomplete postzygotic reproductive isolation. © 1997 The Society for the Study of Evolution.