My research interests lie within the broad field of Behavioural Ecology, and more specifically to the area of mammalian sexual selection. At the beginning of my career I utilized the powerful technique of experimental evolution to explore the evolutionary implications of postcopulatory sexual selection in mice. I established and maintained selection lines of mice, and forced them to breed under different selection regimes to create laboratory populations that were evolving either with (polygamy) or without (monogamy) the influence of the postcopulatory selective force of sperm competition. I observed divergence in sperm competition traits between males from the different populations after only a few generations of selection. As predicted, males evolving with sperm competition had improved fertility compared to males that had evolved in the absence of sperm competition. At the same time, I sampled island populations of mice, quantified the level of sperm competition and male fertility within these populations, and found similar results to what I had observed within the laboratory. Most recently, my research has focused on interactions between the gametes and the implications of such interactions for sexual selection, sexual conflict and speciation. In particular, I took advantage of the specialized technique of in vitro fertilization to explore the phenomenon of ‘egg defensiveness’. An earlier study of mine had shown that males with high quality sperm have greater in vivo fertilization success, and suggested that competitive conditions will favour sperm that rapidly penetrate the egg. However, a detrimental outcome of increased ‘sperm aggressiveness’ is an elevation in the frequency that more than one sperm will enter the egg (polyspermy). As a response to the fatal threat of polyspermy, females are expected to counter-adapt by increasing their resistance to fertilization, or become more ‘defensive’. Sexual conflict theory thus views females as walking an evolutionary tightrope: reduced defenses increase the risk of polyspermy, while overly efficient defenses may prevent fertilization altogether. Empirical support of this paradigm was previously limited to just two comparative studies. My recent investigations have extended this evidence beyond correlational data, and for the first time within a species, I have observed both evolutionary and plastic responses in egg resistance to fertilization. These results have shown that sperm competition has the potential to drive egg resistance to fertilization, which may rapidly generate reproductive barriers between populations.