Kerr, Benjamin, and Joshua Nahum. “The evolution of restraint in structured populations: setting the stage for an egalitarian major transition.” The major transitions in evolution revisited. MIT Press, Cambridge (2011): 127-140.
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The prolific reef-building capacity of hermatypic corals depends on their association with single-celled photosynthetic endosymbionts called zooxanthellae (Knowlton 2001). One might think of the reef itself as a magnificent signature of an interspecific union. A “major transition” in evolution has transpired: Formerly independent entities have come to rely on one another as a “higher-level” entity for continued existence. If we dig a bit deeper evolutionarily, the coral contains a layered series of such transitions. There is evidence that a nonphotosynthetic ancestor of zooxanthellae engulfed a red algal cell, which eventually generated its photosynthetic plastid (Bhattacharya et al. 2007; Keeling 2004). And this was not the first of such maneuvers: A nonphotosynthetic ancestor to the red algae engulfed a cyanobacterium, eventually giving rise to the algal chloroplast. Furthermore, the mitochondria in both coral cells and zooxanthellae come from a proteobacterium that was engulfed by an ancestor common to these present-day symbionts (Emelyanov 2003). Thus, the coral is an elaborate story of serial and parallel symbiotic transitions.