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From the 1960s through the early 1990s, substantive and far-reaching changes in population genetics occurred. The debates concern the units of selection, and changed the nature and scope of evolutionary explanations, turning attention from genotypic models to models of structured populations. Furthermore, the relation of a trait to its environment was explored in both models and empirical investigations. For example, Maynard Smith's early theoretical work on group selection provoked theoretical debate, whereas Michael Wade and colleagues produced empirical evidence for the efficacy of group selection as an evolutionary component. Wade's (1978) paper, in particular, criticized ill-conceived, oversold, and rigid theoretical requirements for the efficacy of group selection in nature. The result was a blossoming in a corner of population biology – both in theoretical genetics and empirical studies – of investigations into the possibility of higher-level interactors such as those that had been found in the house mouse by Lewontin and Dunn (1960, 1962) (reviewed in Section 4; for detailed analysis of structured population models, see Lloyd 1988/1994). Various attempts had been made to expand population genetics beyond the organismically focused efforts of the pre-1960s, but the work of the 1980s is distinguished by its achievement of consensus about many fundamentals of hierarchical population genetics and by its empirical substantiation of these ideas. Philosophers of biology soon joined the revolution, sorting out the various disputes and definitions, analyzing the variety of evolutionary models, focusing in on what they saw as the key issue of contention: how could the various methods and principles be used for sorting out the key “units of selection” – the objects directly involved in selection processes determining the success of genes, thus engaging in “the interactor debates.”