 On this analysis, the different stages of an evolutionary transition involve different conceptions of multi-level selection (Okasha 2006, 2015). Using the distinction defended by Lorraine Heisler and John Damuth (Heisler &amp; Damuth 1987; Damuth &amp; Heisler 1988) in their “contextual analysis” of units of selection, this analysis claims that early on in the process of an evolutionary transition, multi-level selection 1 (MLS1), in which the particles themselves are the “‘focal’ units” upon which selection directly acts, applies. However, by the end of the transition, both the collectives and the particles are focal units of selection processes, with independent fitnesses, a case of Damuth and Heisler’s multi-level selection 2 (MLS2) (Okasha 2006: 4). An easy way to capture this distinction is that, under MLS1, the lower level particles are the interactors as well as the replicators, while in MLS2, both the upper level collectives as well as the particles are interactors. Thus, the issues surrounding evolutionary transitions involve both the interactor question and the replicator question. Understanding evolutionary transitions hence provides additional significance to Damuth and Heisler’s distinction: 