 In our preceding discussions of units of selection, we have restricted ourselves to situations in which the various units were pre-established entities. Our approach has been synchronic, one in which the relevant units, be they genes, organisms, or populations, are the same both before and after a given evolutionary process. However, not all evolutionary processes may be able to be captured under such a perspective. In particular, recent discussions regarding so-called “evolutionary transitions” present a unique complication to the debates over units and levels of selection.