The light-producing chemistry behind bioluminescence varies across the lineages of bioluminescent organisms. Based on this observation, bioluminescence is believed to have evolved independently at least 40 times. In bioluminescent bacteria, the reclassification of the members ofVibrio fischeri species group as a new genus, Aliivibrio, has led to increased interest in the evolutionary origins of bioluminescence. Among bacteria, the distribution of bioluminescent species is polyphyletic. For instance, while all species in the terrestrial genus Photorhabdus are luminescent, the genera Aliivibrio, Photobacterium, Shewanella and Vibrio contain both luminous and non-luminous species. Despite bioluminescence in bacteria not sharing a common origin, they all share a gene sequence in common. The appearance of the highly conserved lux operon in bacteria from very different ecological niches suggests a strong selective advantage despite the high energetic costs of producing light. DNA repair is thought to be the initial selective advantage for light production in bacteria. Consequently, the lux operon may have been lost in bacteria that evolved more efficient DNA repair systems but retained in those where visible light became a selective advantage. The evolution of quorum sensing is believed to have afforded further selective advantage for light production. Quorum sensing allows bacteria to conserve energy by ensuring that they do not synthesize light-producing chemicals unless a sufficient concentration are present to be visible.