THE ROLE OF SYMBIOSIS IN ADAPTATION AND S PECIATION

Symbiosis is an association, at least during a part of the life cycle, between two or several specifically distinct organisms. This associat ion, completed by the coadaptation and the coevolution of partners, le ads to the formation of a new biological entity, the symbiocosme, itse lf submitted to natural selection. Concerning physiology, symbiosis ge nerally improves the degree of adaptation to the environment. From a g enetical point of view, symbiosis is a sophisticated mechanism with wh ich to acquire new genes by lateral transfer. The integration of symbi otes to the host metabolism is sometimes so high that the symbiote loo ks like a cytoplasmic organelle, as in the weevil Sitophilus oryzae. T his is an argument in favor of the serial endosymbiosis theory, accord ing to which mitochondria and plasts would he of symbiotic origin. Con cerning speciation, it could he facilitated by gene transfer and by th e incompatibility phenomenon which leads to reproductive isolation.