 Evolutionary theorists often distinguish between the unit of selection (Williams 1966; Franklin &amp; Lewontin 1970; Lewontin 1970) and the unit of evolution (Williams 1966; Hull 1978; Ereshefsky 1991). Expanding the category of what can act as a unit of selection leads directly into a corresponding expansion of what can act as a unit of evolution. This is how a hierarchical theory of evolution is theoretically generated: when theoreticians consider not just organisms or individuals as the  unit of selection (see entry)  but also species and (kin) groups, and genes (for instance), they are also and correspondingly considering not just species or populations as the unit of evolution but also clades, and cultures, and cells or perhaps organisms (respectively). Each proposed unit of selection incurs another, higher-level candidate for the unit of evolution. Hierarchical evolutionary theory is just the name for theories of evolution that admit of different units of selection and evolution than that of (merely and respectively) organisms or individuals and species or populations. Hierarchical theorists see evolutionary processes (especially selection processes) unfolding simultaneously at different biological levels and temporal scales.