When thinking about biological evolution, we usually imagine thousands of years over which animals and plant species change.  With microbes however, significant changes can happen in the matter of days or a few moments.  In the marine environment for example, as much as a thousand different variants of a single microbial species can be found in a milliliter of seawater, with hundreds or thousands of species present. We ask two main questions: How does this diversity arise ? What role does it play in nature ? To answer such broad questions, we study evolutionary processes in the Vibrios, a group of bacteria ubiquitous in aquatic environments, responsible for  diseases as diverse as bleaching in corals and cholera in humans. We use techniques from the fields of population genetics, molecular epidemiology as well as experimental