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Arne Traulsen studies evolutionary game theory and evolutionary dynamics in finite biological populations. His work uses mathematical models to separate the effects of individual interactions, population structure, and chance in natural selection.^[ref]

With Martin A. Nowak, Traulsen constructed a 2006 model of group selection in which only individuals reproduced. Offspring stayed in the parent's group; a group reaching a size limit could split, displacing another group. Cooperation could lower an individual's success against nonhelpers within its group while increasing its group's rate of growth and splitting.^[ref]

The model supplies an illuminating distinction between levels of description. A trait's frequency can fall within mixed groups yet rise in the combined biological population when more cooperative groups contribute more descendants. Specifying the reproduction and splitting rules made this proposal testable by calculation and simulation, rather than inferring an evolutionary advantage solely from a benefit to the group.^[ref]

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.

Group selection is a contribution to natural selection arising because groups differ in their survival, growth or production of descendant groups. A heritable trait can spread through greater success of groups containing it even while its frequency declines within each mixed group. For cooperation, the essential question is whether this difference between groups can overcome an individual's cost in biological fitness.

Consider a hypothetical biological population with two groups of 100 individuals: one contains 80 cooperators and the other 20. Initially, the overall cooperative fraction is $100/200=0.50$. Suppose the first group grows to 200 individuals with 75% cooperators, while the second remains at 100 with 15% cooperators. Cooperation has declined within both groups, yet the combined cooperative fraction isThe more cooperative group now contributes more individuals to the total. These invented counts illustrate why within-group change and changes in group weights must both be measured; they do not establish the cause of a real group's growth.

In a 2006 model, Arne Traulsen and Martin A. Nowak made this mechanism explicit: individuals reproduced within groups, groups could split on reaching a size limit, and a split displaced another group. Groups with more cooperators could split more often despite the within-group advantage of withholding help. The population structure and rules governing group persistence therefore mattered alongside individual biological fitness.^[ref]