Males produce a myriad of proteins that are transferred to the female during mating and directly influence reproductive success. As a group, the genes that encode these male reproductive proteins exhibit elevated levels genetic diversity and rapid evolution. The proposed research aims to further understand the evolutionary causes and consequences of the rapid evolution of male reproductive proteins, a ubiquitous but enigmatic pattern. Using the red flour beetle as a model system, this research will combine both genetic and phenotypic approaches to differentiate between two alternate hypotheses proposed to explain the rapid evolution observed among this class of genes. Specifically, the research aims to determine whether the accumulation of genetic variation is due to adaptive or neutral evolutionary forces. This research will also benefit a number of undergraduate students who will be trained by the graduate student co-PI. The co-PI also contributes to a popular science blog accessible to a general audience. Furthermore, understanding the evolution of male reproductive proteins will inform areas of applied research. For example, efforts aimed at developing biological control methods for insect disease vectors and agricultural pests, such as the red flour beetle, have recently begun to investigate whether this group of genes may provide new avenues for population control.