Conflict, cheats and the persistence of symbioses.

Many symbioses are widespread, abundant, and evolutionarily persistent. This is despite unambiguous evidence for conflict between the partners and the existence of cheats that use benefits derived from their partners while providing reduced or no services in return. Evidence from a diversity of associations suggests that symbioses are robust to cheating in several ways. Some symbioses persist despite conflict and cheating because of the selective advantage of cost-free interactions (also known as byproduct mutualistic interactions), which incur no conflict. There is also evidence for the suppression of cheating by sanctions imposed by partners in some symbioses, and vertical transmission has been shown experimentally to promote traits that enhance partner performance. It is argued that these processes contribute to the apparent rarity of evolutionary transitions from symbiosis to parasitism. There is strong phylogenetic evidence for the evolutionary reversion of various symbiotic organisms to free-living lifestyles, but at least some of these transitions can be attributed to selection pressures other than within-symbiosis conflict. The principal conclusion is that, although conflict is common in symbioses, it is generally managed and contained.