Relaxed selection in the wild.

Natural populations often experience the weakening or removal of a source of selection that had been important in the maintenance of one or more traits. Here we refer to these situations as 'relaxed selection,' and review recent studies that explore the effects of such changes on traits in their ecological contexts. In a few systems, such as the loss of armor in stickleback, the genetic, developmental and ecological bases of trait evolution are being discovered. These results yield insights into whether and how fast a trait is reduced or lost under relaxed selection. We provide a prospectus and a framework for understanding relaxed selection and trait loss in natural populations. We also examine its implications for applied issues, such as antibiotic resistance and the success of invasive species.