It is well known that most organisms are composed of smaller subunits, including not only cells and semi-independent organelles but often also a whole suite of parasites, commensal organisms, and other symbionts (Buss 1987; Margulis 1970; Bouchard andHuneman 2013). The process of association of these subunits has been recognized as contributing to many of the major transitions that have occurred in the course of evolution (Maynard Smith and Szathmary 1995; Michod 1999). As a result, the selective forces that have led to the subunits foregoing their evolutionary sovereignty are being better appreciated (van Baalen and Jansen 2001), but little is known yet about the mechanisms that maintain the dynamic coherence of the associations. In this respect there are many parallels with classical ecological questions. In spite of the fact that the processes that dynamically generate and maintain biodiversity in ecosystems (competition, herbivory, predation, parasitism, and so forth) are quite well known, westill have difficulty in understanding and predicting the outcome.
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