Horizontally acquired mutualisms, an unsolved problem in ecology?

Current models of the evolution and maintenance of mutualism stress the importance of vertical transmission and the genetic uniformity of symbionts to evolutionary stability. Here we point out that many well-known mutualistic relationships involve symbionts that are neither vertically transmitted nor genetically uniform. We have therefore re-examined some of the existing models in an attempt to identify general conditions under which horizontally acquired mutualisms might be evolutionarily stable.