  How cooperation emerges in human societies is still a puzzle. Evolutionary
game theory has been the standard framework to address this issue. In most
models, every individual plays with all others, and then reproduce and die
according to what they earn. This amounts to assuming that selection takes
place at a slow pace with respect to the interaction time scale. We show that,
quite generally, if selection speeds up, the evolution outcome changes
dramatically. Thus, in games such as Harmony, where cooperation is the only
equilibrium and the only rational outcome, rapid selection leads to dominance
of defectors. Similar non trivial phenomena arise in other binary games and
even in more complicated settings such as the Ultimatum game. We conclude that
the rate of selection is a key element to understand and model the emergence of
cooperation, and one that has so far been overlooked.
