My research focuses on two interlinked topics. Firstly, I have been working on various aspects of sexual selection in a socially monogamous species with extra-pair paternity, the blue tit. I found evidence for benefits of individual genetic variation, and for mate choice mechanisms that help increasing offspring genetic variation. Secondly, I started to use quantitative genetic methods to study mechanisms that may maintain genetic variation in natural populations. Heritable genetic variation for fitness traits is a paradox finding, given our understanding of directional selection processes on such traits. I explored differential selection on male and female fitness traits in red deer and found sexually antagonistic genetic variation for fitness in this species. I am now continuing to study antagonistic selection on male and female individuals, using data from the white campion.