Fig 5. Evolutionary dynamics of the Rap-Phr pathway.

Rap-Phr expansion combines a horizontal mode of acquisition—systems diverge in separate lineages and then recombine by HGT—with a vertical mode of divergence—an autoinducer is duplicated and then one of the duplicates is modified. Social interactions govern both diversifying selection between lineages and acquisition of a diverged system [41,60,61]. Acquisition by horizontal transfer is sometimes followed by fixation in the lineage.