The evolution and maintenance of cooperation in natural and artificial populations

Cooperative behaviors, where an individual benefits another at expense to its self, pose a special problem for biologists. While populations of cooperators have higher average fitness than populations of uncooperative individuals, cooperative populations are open to invasion by cheaters that defect (do not cooperate) when interacting with cooperative individuals. I first describe the relationships between on-territory and off-territory reproduction in red-winged blackbirds. I then show how extra-pair paternity can influence two cooperative behaviors in red-winged blackbirds, reduced territorial aggression among neighbors and cooperative nest defense. Finally, I use genetic algorithms to perform simulations to explore the relationship between population structure and the evolution of cooperation.; Reduced aggression among territorial males is potentially a form of cooperation because individuals are tempted to cheat by unilaterally expanding their territory or by seeking extra-pair copulations on their neighbor's territory. I used simulated defections to determine if reduced aggression was a form of reciprocal altruism. I used paternity analysis coupled with behavioral observations of territorial behavior to estimate the frequency of cheating and to determine if males respond to cuckoldry by their neighbors. The simulated defections showed that males responded to simulated defections by increasing territorial aggression toward the neighbor, a result indicative of reciprocal altruism. My results also show that males are more aggressive towards males that have successfully cuckolded them. Additional evidence suggests that males may be able to assess a neighbor's ability to cuckold rather than directly detect cuckoldry.; I also used simulated defections combined with paternity analysis of nestlings to simultaneously test between reciprocal altruism and by-product mutualism. Previous studies have provided evidence for either hypothesis although no study has tested for both in the same population. The results from my study show that in this population males participate in cooperative nest defense primarily as a form of reciprocal altruism. There was no evidence of by-product mutualism in the form of males defending nests on other territories in which they had obtained extra-pair fertilizations.; Simulations using genetic algorithms show that population structure enhances the evolution of cooperation from an uncooperative population. Populations composed of small subpopulations achieved higher rates of cooperation than populations composed of large subpopulations. Additionally, population structure influenced the strategies that evolved. The common strategies in the smallest populations cooperated almost unconditionally. The most common strategies in lightly larger populations, however, were similar to Tit-For-Tat.