= Group selection
<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 is
$$
\frac{200(0.75)+100(0.15)}{200+100}=0.55.
$$
The 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>.<Evolution of cooperation by multilevel selection by Arne Traulsen and Martin A. Nowak>[PNAS 103, pp. 10952–10955; abstract, Figure 1 and model definition]{ref}
Back to article page
