"Antagonistic coevolution between hosts and parasites has far-reaching consequences for evolutionary ecology, agriculture and medicine. Coevolution between bacteria and virulent phages is likely to be of particularly broad significance because of the ubiquity of phages, the key role played by bacteria in ecosystem functioning, and the therapeutic use of phages as ‘evolving’ antibiotics, in both clinical and agricultural contexts. Despite the far-reaching consequences of bacteria-phage coevolution in laboratory populations, we know virtually nothing about its operation and significance in natural ecosystems. Two complimentary approaches will be taken in order to increase our understanding of evolutionary maintenance of bacteria and phages in soil, and coevolutionary dynamics within communities in natural environments. First, I will determine the significance of coevolution for driving the within-population diversity of the focal bacteria population, and in turn how changes in the focal bacterial population affects community composition, at both phenotypic and genetic levels (Objective 1). In addition, I will assess the impact of coevolution on the evolution of bacteria mutation rate, and whether phages can select for increased mutation rates in soil environments (Objective 2). Furthermore, I will determine the extent to which natural selection acts on bacterial cooperative in soil, and how these are affected by phages (Objective 3). I will also assess the impact of some ecologically relevant manipulations on the rate of coevolution (Objective 4). The second approach will allow entirely natural communities to be studied. I will directly measure coevolution between natural populations of Pseudomonas-like bacteria and associated phages in soil, by measuring interactions across time (Objective 5). This proposal will provide a crucial understanding of the role of the phages in driving ecological and evolutionary changes of bacteria in soil environments."