 Evolution of cooperation by multilevel selection by Arne Traulsen and Martin A. Nowak

By Arne Traulsen and Martin A. Nowak. PNAS 103(29), 10952–10955 (18 July 2006), DOI doi.org/10.1073/pnas.0602530103

Original paper: dash.harvard.edu/bitstreams/9473ec8f-4d25-4d08-89ba-6eb725093f5e/download

Abstract and Figure 1: reproduction within groups, splitting at a size threshold and elimination of another group. The simplified threshold assumes weak selection, rare splitting and many groups.