 The gene’s-eye formulation of natural selection sparked an explosion of philosophical attention to evolutionary biology (which happened to roughly coincide with the attention to the relationship between classic and molecular genetics discussed in  section 2.1).  This “levels” or “units of selection” debate revolved around a number of questions (Keller 1999; Okasha 2006; entry:  units and levels of selection).  For example, at what level does natural selection act? Genes, individuals, groups, entire species (entry:  biological notion of individual)?  Group selectionists argued altruistic behavior could arise if one group was fitter than another group because it had more altruistic members in that group (Sober and Wilson 1998). Stephen Jay Gould made the case for selection acting on entire species when, for example, ecological specialists were more common than generalists (Gould 2002). Other philosophers questioned the entire replicator/interactor distinction; Developmental Systems Theorists advocated instead for seeing the developmental system as a whole (genes and the environment) as the evolving unit (Oyama, Griffiths, and Gray eds. 2001; see the entry  on developmental biology).