A series of selection experiments with Tribolium castaneum (the flour beetle) as the experimental organism are being used to investigate the nature of quantitative genetic variation in segregating diploid populations. Emphasis in the project has centered on the following issues: (a) the importance of overdominant genes in the inheritance of quantitative traits (b) the number of genes responsible for the genetic variation (c) the relationship of variation in metric traits to the maintenance of polymorphism in population and (d) the role of multiple peak epistasis in determining selection limits. A new aspect of the proposal will include an evaluation of the effects of long term selection on changing total genome content (gene multiplicity) of the populations in addition to its affect on allelic frequency changes. A major component of the work to date has been the use of directional mass selection for pupa weight as a tool for creating a series of plateaued populations to test for the amount and nature of residual genetic variation. Statistical analyses of variation in the plateaued populations combined with inbreeding experiments and selection for combining ability are providing information on items (a) through (c) above. A series of selection experiments where the foundation populations are crosses and backcrosses of inbred lines derived from the plateaued populations will be used to investigate multiple peak epistasis, an important component of Wright's shifting balance theory of evolution. Finally the plateaued population provide us with unique experimental material to investigate whether long term selection for rapid growth has altered the amount of ribosomal DNA in the populations. RNA-DNA hybridization studies will be used to compare the fraction the total DNA content that is ribosomal DNA in the plateaued and unselected populations. If differences are found, a series of experiments are planned to determine how much of the total response to selection can be attributed to changes in ribosomal DNA as compared with allelic frequency changes at other loci.