  Cooperation is beneficial for the species as a whole, but, at the level of an
individual, defection pays off. Natural selection is then expected to favor
defectors and eliminate cooperation. This prediction is in stark contrast with
the abundance of cooperation at all levels of biological systems: from cells
cooperating to form a biofilm or an organism to ecosystems and human societies.
Several explanations have been proposed to resolve this paradox, including
direct reciprocity, kin, and group selection. However, our work builds upon an
observation that selection on cooperators might depend both on their relative
frequency in the population and on the population density. We find that this
feedback between the population and evolutionary dynamics can substantially
increase the frequency of cooperators at the front of an expanding population,
and can even lead to a splitting of cooperators from defectors. After
splitting, only cooperators colonize new territories, while defectors slowly
invade them from behind. Since range expansions are very common in nature, our
work provides a new explanation of the maintenance of cooperation.
