The role of symbiosis in adaptation and speciation

Symbiosis is an association, at least during a part of the life cycle, between two or several specifically distinct organisms. This association, completed by the coadaptation and the coevolution of partners, leads to the formation of a new biological entity, the symbiocosme, itself submitted to natural selection. Concerning physiology, symbiosis generally improves the degree of adaptation to the environment. From a genetical point of view, symbiosis is a sophisticated mechanism with which to acquire new genes by lateral transfer. The integration of symbiotes to the host metabolism is sometimes so high that the symbiote looks like a cytoplasmic organelle, as in the weevil Sitophilus oryzae. This is an argument in favor of the serial endosymbiosis theory, according to which mitochondria and plasts would he of symbiotic origin. Concerning speciation, it could he facilitated by gene transfer and by the incompatibility phenomenon which leads to reproductive isolation.