Sexual selection is known to favor male traits that are beneficial to their bearers but are collaterally harmful to their mates. It has been suggested that within-species sexual selection and consequent harmful male traits may strengthen interspecific reproductive interference, yet the evidence is circumstantial. We tested this hypothesis by employing an experimental evolution, where the seed beetles Callosobruchus chinensis were kept in either monogamy or polygamy for 17 generations. Monogamous mating eliminates the opportunity for sexual selection and reduces the extent of sexual conflict, whereas polygamous mating retains both. Callosobruchus chinensis from polygamous lines were larger than those from monogamous lines, demonstrating the evolutionary responses to sexual selection. Furthermore, C. chinensis males of polygamous lines interfered more severely with the reproduction of females of another species C. maculatus than males of monogamous lines. Thus, our results support the hypothesis that sexual selection within a species can result in more intensive reproductive interference between species.