Recombination is a genetic consequence of sexual reproduction; it results in chromosomes containing genes from both parents. The goal of this project is to understand how natural selection can act on recombination. The form of genetic variation for recombination determines how the response to selection can occur. This variation will be studied in a population of the fruit fly Drosophila melanogaster. Genes on the X and third chromosomes will be studied to see how they influence recombination in the X chromosome. This will show the form of the variation for recombination due to both linked and unlinked modifier genes, and how they interact. Theoretical analysis will aid the interpretation of the experimental results. Mathematical analysis and simulations will be done for models of multiple recombination modifiers. The goal is to see how the net selection on recombination is related to the selective effects from the individual modifiers.