The fate of cooperation during range expansions

TL;DR

This paper explores how range expansions can promote cooperation by enabling cooperators to split from defectors and colonize new territories, offering a new explanation for the persistence of cooperation in nature.

Contribution

It demonstrates that population density and frequency-dependent selection during range expansions can significantly enhance cooperation and lead to cooperator-defector splitting.

Findings

Cooperators can increase in frequency at the expansion front.

Splitting of cooperators from defectors can occur during range expansion.

Only cooperators colonize new territories, defectors invade later.

Abstract

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.