 Explaining evolutionary transitions in Darwinian terms poses a particular challenge: “Why was it advantageous for the lower-level units to sacrifice their individuality and form themselves into a corporate body?” (Okasha 2006: 218). On one analysis, three stages of such a transition, each defined in terms of the connection between fitness at the level of the collective and the individual fitness of its component particles, are identified (Okasha 2006: 238). Initially, collective fitness is simply defined as average particle fitness. As fitness at the two levels begins to be decoupled, collective fitness remains proportional to average particle fitness, but is not defined by it; at such a stage, “the emerging collective lacks ‘individuality’, and has no collective-level functions of its own” (Okasha 2006: 237). Finally, collective fitness “starts to depend on the functionality of the collective itself” (Okasha 2006: 237–8; see Okasha 2015 for a representation of this in terms of causal graphs).