群体结构促进直接互惠中具有长记忆的伙伴型与竞争型策略的共存
Grouping promotes both partnership and rivalry with long memory in direct reciprocity
AI总结：
本研究通过演化模拟对比不同记忆长度的策略在两类种群中的演化动态，发现群体结构与长记忆的相互作用可使友好竞争型策略占据主导，大幅提升合作水平。
摘要：
生物学家和社会科学家长期以来一直关注如何在重复囚徒困境中协调个体利益与集体利益的问题。学界已提出诸多有效策略，通常被划分为“伙伴型（partners）”和“竞争型（rivals）”两大类。近期，在更长记忆的策略空间中，研究者发现了第三类策略——“友好竞争型（friendly rivals）”。这类策略同时具备伙伴型和竞争型的特征：它们与同类博弈时会完全合作，如同伙伴型策略；但又绝不会让对手获得更高收益，如同竞争型策略。尽管这类策略具有颇具吸引力的理论特性，但目前尚不清楚它们能否在演化种群中涌现，因为此前的多数研究都聚焦于记忆-1（memory-one）策略空间，而该空间中不存在友好竞争型策略。为探究这一问题，我们在均匀混合种群和群体结构种群中开展了大规模演化模拟，对比了记忆-1与记忆-3（memory-three）策略空间的演化动力学。在均匀混合种群中，记忆长度未产生显著差异，种群规模和合作收益是核心影响因素；友好竞争型策略仅发挥次要作用，因为在给定环境中，成为伙伴型或竞争型策略通常已足够。而在群体结构种群中，记忆长度的影响极为显著：当存在记忆-3策略时，友好竞争型策略会占据主导，合作水平几乎达到最大值，即便合作收益低到在均匀混合种群中无法实现合作的程度亦是如此。这一结果凸显了群体结构与记忆长度之间的相互作用对合作演化的重要驱动作用。
英文摘要：
Biological and social scientists have long been interested in understanding how to reconcile individual and collective interests in iterated Prisoner's Dilemma. Many effective strategies have been proposed, and they are often categorized into one of two classes, `partners' and `rivals.' More recently, another class, `friendly rivals,' has been identified in longer-memory strategy spaces. Friendly rivals qualify as both partners and rivals: They fully cooperate with themselves, like partners, but never allow their co-players to earn higher payoffs, like rivals. Although they have appealing theoretical properties, it is unclear whether they would emerge in evolving population because most previous works focus on memory-one strategy space, where no friendly rival strategy exists. To investigate this issue, we have conducted large-scale evolutionary simulations in well-mixed and group-structured populations and compared the evolutionary dynamics between memory-one and memory-three strategy spaces. In a well-mixed population, the memory length does not make a major difference, and the key factors are the population size and the benefit of cooperation. Friendly rivals play a minor role because being a partner or a rival is often good enough in a given environment. It is in a group-structured population that memory length makes a stark difference: When memory-three strategies are available, friendly rivals become dominant, and the cooperation level nearly reaches a maximum, even when the benefit of cooperation is so low that cooperation would not be achieved in a well-mixed population. This result highlights the important interaction between group structure and memory lengths that drive the evolution of cooperation.
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