"Horizontal gene transfer (HGT) is a fundamental process of bacterial evolution, accelerating adaptation to novel environments and providing access to new ecological niches. However, two of the three mechanisms of HGT, transduction and conjugation, both rely on semi-autonomous vectors (lysogenic phages and conjugative plasmids, respectively), creating the potential for coadaptation between microbe and vector. I here focus on conjugative plasmids. These encode their own replication and transfer, and as such are capable of pursuing their own fitness interests, which need not be aligned with those of their bacterial host. My thesis is that bacterial adaptation by conjugation must therefore be viewed as a co-evolutionary, rather than simply an evolutionary process as achieved to date. In this proposal I take an experimental evolution approach to derive an empirically founded understanding of bacteria-plasmid coevolutionary processes. In particular, I focus on the effects (on the pattern and process of bacteria-plasmid coevolution) of ecological variables identified in population models as crucial to the persistence of conjugative plasmids: environmental heterogeneity, spatial structure, and between-species transfer. Drawing on coevolutionary theory, I highlight that the ecological conditions expected to favour plasmid persistence may often drive the breakdown of bacteria-plasmid coadaptation. Additionally, I will determine the consequences of bacteria-plasmid coevolution for the structuring of microbial communities."