The edge of neutral evolution in social dilemmas

The functioning of animal as well as human societies fundamentally
relies on cooperation. Yet, defection is often favorable for the
selfish individual, and social dilemmas arise. Selection by
individuals' fitness, usually the basic driving force of evolution,
quickly eliminates cooperators. However, evolution is also governed
by fluctuations that can be of greater importance than fitness
differences, and can render evolution effectively neutral. Here, we
investigate the effects of selection versus fluctuations in social
dilemmas. By studying the mean extinction times of cooperators and
defectors, a variable sensitive to fluctuations, we are able to
identify and quantify an emerging 'edge of neutral evolution' that
delineates regimes of neutral and Darwinian evolution. Our results
reveal that cooperation is significantly maintained in the neutral
regimes. In contrast, the classical predictions of evolutionary
game theory, where defectors beat cooperators, are recovered in the
Darwinian regimes. Our studies demonstrate that fluctuations can
provide a surprisingly simple way to partly resolve social
dilemmas. Our methods are generally applicable to estimate the role
of random drift in evolutionary dynamics.