ON THE INSTABILITY OF EVOLUTIONARY STABLE STRATEGIES IN SMALL POPULATIONS

Evolutionary stable strategies (ESSs) are often used to explain the be haviors of individuals and species. The analysis of ESSs determines wh ich, if any, combinations of behaviors cannot be invaded by alternativ e strategies. Two assumptions required to generate an ESS (i.e. an inf inite population and payoffs described only on the average) do not hol d under natural conditions. Previous experiments have indicated that u nder more realistic conditions of finite populations and stochastic pa yoffs, populations may evolve in trajectories that are unrelated to an ESS, even in very simple evolutionary games. The simulations are exte nded here to small populations with varying levels of selection pressu re and mixing levels. The results suggest that ESSs may not provide a good explanation of the behavior of small populations even at relative ly low levels of selection pressure and even under persistent mixing. The implications of these results are discussed briefly in light of pr evious literature which claimed that ESSs generated suitable explanati ons of real-world data. (C) 1998 Elsevier Science B.V. All rights rese rved.