Costs of resistance limit the effectiveness of cooperation enforcement

Cooperative groups are susceptible to invasion by cheaters that reap the benefits but fail to pay the costs. Both theory and experimental work have shown that cheating can select for counter-adaptations to resist cheating. But then why is cheating so common? One key hypothesis is that trade-offs prevent resistors from taking over, but evidence to support the trade-off model is lacking. Here we evolved resistance to different cheaters and tested for trade-offs. Improvements against one cheater frequently entailed correlated improvements against novel cheaters. However, direct responses to selection were typically stronger than correlated responses to selection, resulting in a pattern of local adaptation of resistance. Control populations, evolved in the absence of a cheater, showed improvements in spore germination, whereas cheater-evolved populations did not, suggesting that the evolution of resistance retards or prevents other fitness improvements. Taken together, our findings suggest that, although cheater resistance can evolve rapidly, it may also involve subtle trade-offs that can help to explain the maintenance of polymorphism in cheating and resistance in nature.