 Philosophers have also contended that particular terms in models of systems featuring the formation of groups (or collectives) should be understood as quantifying the influence of selection at different levels. So, some type recursions of systems involving groups feature “group fitness” or “collective fitness” parameters, ones analogous to individual or particle fitness parameters. Kerr and Godfrey-Smith (2002) discuss one such system of recursions; Jantzen (2019) defends an alternative parameterization of group selection as part of different system of equations. (See also Krupp 2016 for causal-graphical conceptualization of the notion of group selection.) For much more on multi-level selection, see entry on  units and levels of selection.