The parallel relations between a phylogeny of insect species based on mitochondrial genes (the parasitoid species Nasonia and their host fly (Sarcophaga bullata) and a dendrogram of the microbial community relations based on a bacterial gene. Note the close correspondence between the phylogeny of the two lineages. Source.
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!
Some of the examples they cite are startling. For one, Wolbachia, like some other bacterial symbionts of insects, induces parthenogenesis in the insect host, a form of asexual reproduction that does not involve fertilization and leads to what is called “asexual speciation”. Click here and here for some of our previous posts on Wolbachia. For another, Drosophila flies reared on different diets house different microbiota, and show strong mating discrimination; ergo, the symbionts dictate who mates with whom. Further, bacterial symbionts that we could classify as vertically-transmitted, nutritional mutualists (e.g., insect symbionts in the genus Buchnera) assist in resource exploitation, thereby creating new ecological opportunities for their host. In some cases, host and endosymbiont appear to have evolved together. There is even more. Endosymbionts can also induce cytoplasmic incompatibility. Here the offspring of infected males and uninfected females are sterile, therefore, unproductive. In other cases, the offspring of hybrid matings become more susceptible to infection than non-hybrids, which may reduce their fertility and viability. How all of these examples of symbioses affect speciation is discussed in detail in the article.