A fundamental aim in biology is to understand the (co)evolutionary dynamics of the adaptive reproductive strategies that translate ecology into evolution. However, until now, it has not been possible to explicitly test key hypotheses explaining the evolution of major reproductive strategies in wild populations experiencing real-life ecological variation. I will revolutionise our understanding of the (co)evolution of two fundamental reproductive strategies, and our approach to achieving such understanding, by deriving entirely new evolutionary quantitative genetic theory and providing the first explicit tests of this theory in nature.Genetic polyandry (female reproduction with multiple males) and inbreeding (reproduction among relatives) are fundamental reproductive strategies that profoundly influence the social, genetic and genomic structures of populations. Yet decades of research have failed to explain their (co)evolution and persistence in the face of sexually antagonistic selection. Current theory is inadequate because it does not consider ecology or coevolution or make critical quantitative predictions that permit definitive test of key hypotheses in wild populations. Key forces of direct and indirect selection on genetic variation underlying polyandry and inbreeding have consequently never been explicitly estimated.I will derive new theory that defines the (co)evolution of polyandry and inbreeding in terms of sex-specific genetic (co)variances, thereby providing the conceptual advance required to drive a new generation of empirical test. I will estimate these genetic (co)variances through state-of-the-art quantitative genetic analysis of outstanding wild population data, thereby providing the first explicit tests of key hypotheses explaining the (co)evolution of polyandry and inbreeding in nature. I will thereby initiate and implement a new approach to understanding the evolution of reproductive strategies and answer long-standing questions in biology.