A Comparison of Purebred and Crossbred Selection Schemes with Two Populations of Drosophila Pseudoobscura.

HE methods of selection for changing the values of quantitative characters Tmay be classified into two broad categories. One group includes a variety of methods which are aimed at changing individual population performance at the expense of additive genetic variance within the populations. Some familiar methods in this category are mass selection and family selection. Individuals or families are selected on the basis of their own performance for the propagation of the population, and they are never outcrossed with members of other populations. Therefore, this category OI selection will be termed purebred selection. The other category includes the methods of selection which are designed to improve hybrid or crossbred performances of lines or populations. The members of populations or lines are crossed to members of other lines or populations (testers). and are selected on the basis of the performance of their hybrid progenies. In the case of continued selection these are remated with members of their own population for the maintenance of genotypic superiority with respect to the testers. These types of selection will be called crossbred selection. The genetic variation utilized by crossbred selection is the genetic variance associated with members of the parent population in combination with tester genotypes. In the present study responses in two purebred selection programs from two separate base populations are compared with responses in a crossbred selection program in which the same two populations are used as the testers and the tested reciprocally. The relative effectiveness of the two types of selection will be discussed, and the information from the present study will be compared with that from similar studies in Drosophila conducted by other investigators.