Weak selection

Weak selection is a regime of an evolutionary model in which the focal trait or interaction changes biological fitness only slightly relative to a baseline. It permits calculations close to neutral evolution. In one common evolutionary game theory convention, a payoff $P$ changes biological fitness according towhere $w$ controls the contribution of the game. Weak selection requires the payoff-dependent change to be small on the scale of baseline biological fitness.^[ref]

For example, take $w=0.01$ and two competitors with payoffs $P_1=2$ and $P_2=3$. Their fitnesses are $f_1=1.01$ and $f_2=1.02$. If their chances to fill a vacancy are proportional to biological fitness, the second competitor wins with probabilityonly slightly above the neutral value $1/2$. This invented calculation shows the meaning of a small selective bias. A small numerical $w$ would not ensure the same approximation if the payoffs were made enormously larger. Weak selection is also distinct from small biological population size: the strength of genetic drift and the cumulative effect of selection depend on additional features of the model.