 Early species selectionists also eventually recognized the advantages of keeping the interactor question separate from a requirement for an engineering-type adaptation, dropping the former requirement that, in order for species to be units of selection, they must possess species-level adaptations (Vrba 1989). Ultimately, the current widely-accepted definition of species selection is in conformity with a simple interactor interpretation of a unit of selection (Vrba 1989; see Damuth &amp; Heisler 1988; Lloyd 1988 [1994]; Jablonski 2008).