No doubt, one of the defining events in evolution was the acquisition by some primitive cell of bacterial symbionts that became mitochondria and chloroplasts. Symbiosis has, in fact, driven evolution in many other ways and has had a profound effect in the origin of species. For a deeper understanding of how symbioses has affected speciation, a pertinent review by R. M. Brucker and S. Bordenstein is well worth reading. Based on their own research with the insect endosymbiont Wolbachia, the authors describe how symbiosis affects the reproduction of the insect hosts and, ultimately, the fitness of the species. As the authors point out, there is complementary arithmetic in this relationship: symbiosis makes two organisms into one; speciation, one into two!
Click "source" to read more.