Selection, Controlling Factors and Polymorphism

Speciation requires a change in the controlling factors of one or both of the species concerned, and so involves selective forces which vary with population density. The only good field evidence for such selection is Gershenson's on the hamster. The results of the interaction of selection and controlling factors vary with different systems of control. The mechanisms known to be able to maintain a polymorphism are listed. Many of them involve variable selective coefficients, but only one needs selection varying with population density. The use of polymorphism in detecting competition depends on the way the polymorphism is maintained. When the genetic system is independent of population density the usage is straightforward; when it is not, the results of the correlation analysis may only demonstrate the dependence.