SEPARATE SELECTION. ” Plasmids genes are in fact under differential selection while moving through the prokaryotic community, and consequently, they frequently gain and lose genes, revealing a very dynamic organization. This flexibility is mostly due to the abundance of transposable elements they harbor and that facilitate intra- and intermolecular recombination by creating homology regions. Moreover, plasmids can be both vertically and horizontally inherited in a prokaryotic community.”
http://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-10-59