 In its traditional guise, the interactor question is, what units are being actively selected in a process of natural selection? As such, this question is involved in the oldest forms of the units of selection debates (Darwin 1859 [1964], Haldane 1932, Wright 1945). In an early review on “units of selection”, the purpose of the article was claimed as: “to contrast the levels of selection, especially as regards their efficiency as causers of evolutionary change” (Lewontin 1970: 7). Similarly, others assumed that a unit of selection is something that “responds to selective forces as a unit—whether or not this corresponds to a spatially localized deme, family, or population” (Slobodkin &amp; Rapoport 1974: 184).