A simple rule for the evolution of cooperation on graphs and social networks

A fundamental aspect of all biological systems is cooperation. Cooperative interactions are required for many levels of biological organization ranging from single cells to groups of animals. Human society is based to a large extent on mechanisms that promote cooperation. It is well known that in unstructured populations, natural selection favours defectors over cooperators. There is much current interest, however, in studying evolutionary games in structured populations and on graphs. These efforts recognize the fact that who-meets-whom is not random, but determined by spatial relationships or social networks. Here we describe a surprisingly simple rule that is a good approximation for all graphs that we have analysed, including cycles, spatial lattices, random regular graphs, random graphs and scale-free networks: natural selection favours cooperation, if the benefit of the altruistic act, b, divided by the cost, c, exceeds the average number of neighbours, k, which means b/c > k. In this case, cooperation can evolve as a consequence of 'social viscosity' even in the absence of reputation effects or strategic complexity.     

Social diversity promotes the emergence of cooperation in public goods games

Humans often cooperate in public goods games and situations ranging from family issues to global warming. However, evolutionary game theory predicts that the temptation to forgo the public good mostly wins over collective cooperative action, and this is often also seen in economic experiments. Here we show how social diversity provides an escape from this apparent paradox. Up to now, individuals have been treated as equivalent in all respects, in sharp contrast with real-life situations, where diversity is ubiquitous. We introduce social diversity by means of heterogeneous graphs and show that cooperation is promoted by the diversity associated with the number and size of the public goods game in which each individual participates and with the individual contribution to each such game. When social ties follow a scale-free distribution, cooperation is enhanced whenever all individuals are expected to contribute a fixed amount irrespective of the plethora of public goods games in which they engage. Our results may help to explain the emergence of cooperation in the absence of mechanisms based on individual reputation and punishment. Combining social diversity with reputation and punishment will provide instrumental clues on the self-organization of social communities and their economical implications.     

Social evolution: kin preference in a social microbe

Kin recognition helps cooperation to evolve in many animals, but it is uncertain whether microorganisms can also use it to focus altruistic behaviour on relatives. Here we show that the social amoeba Dictyostelium purpureum prefers to form groups with its own kin in situations where some individuals die to assist others. By directing altruism towards kin, D. purpureum should generally avoid the costs of chimaerism experienced by the related D. discoideum.     

Cooperation and competition in pathogenic bacteria

Explaining altruistic cooperation is one of the greatest challenges for evolutionary biology. One solution to this problem is if costly cooperative behaviours are directed towards relatives. This idea of kin selection has been hugely influential and applied widely from microorganisms to vertebrates. However, a problem arises if there is local competition for resources, because this leads to competition between relatives, reducing selection for cooperation. Here we use an experimental evolution approach to test the effect of the scale of competition, and how it interacts with relatedness. The cooperative trait that we examine is the production of siderophores, iron-scavenging agents, in the pathogenic bacterium Pseudomonas aeruginosa. As expected, our results show that higher levels of cooperative siderophore production evolve in the higher relatedness treatments. However, our results also show that more local competition selects for lower levels of siderophore production and that there is a significant interaction between relatedness and the scale of competition, with relatedness having less effect when the scale of competition is more local. More generally, the scale of competition is likely to be of particular importance for the evolution of cooperation in microorganisms, and also the virulence of pathogenic microorganisms, because cooperative traits such as siderophore production have an important role in determining virulence.     

Evolution of novel cooperative swarming in the bacterium Myxococcus xanthus

Cooperation among individuals is necessary for evolutionary transitions to higher levels of biological organization. In such transitions, groups of individuals at one level (such as single cells) cooperate to form selective units at a higher level (such as multicellular organisms). Though the evolution of cooperation is difficult to observe directly in higher eukaryotes, microorganisms do offer such an opportunity. Here we report the evolution of novel cooperative behaviour in experimental lineages of the bacterium Myxococcus xanthus. Wild-type strains of M. xanthus exhibit socially dependent swarming across soft surfaces by a mechanism known as 'S-motility' that requires the presence of extracellular type IV pili. In lineages of M. xanthus unable to make pili, a new mechanistic basis for cooperative swarming evolved. Evolved swarming is mediated, at least in part, by enhanced production of an extracellular fibril matrix that binds cells-and their evolutionary interests-together. Though costly to individuals, fibril production greatly enhanced population expansion in groups of interconnected cells. These results show that fundamental transitions to primitive cooperation can readily occur in bacteria.     

Variation in behaviour promotes cooperation in the Prisoner's Dilemma game

The Prisoner's Dilemma game is widely used to investigate how cooperation between unrelated individuals can evolve by natural selection. In this game, each player can either 'cooperate' (invest in a common good) or 'defect' (exploit the other's investment). If the opponent cooperates, you get R if you cooperate and T if you defect. If the opponent defects, you get S if you cooperate and P if you defect. Here T > R > 0 and P > S, so that 'defect' is the best response to any action by the opponent. Thus in a single play of the game, each player should defect. In our game, a fixed maximum number of rounds of the Prisoner's Dilemma game is played against the same opponent. A standard argument based on working backwards from the last round shows that defection on all rounds is the only stable outcome. In contrast, we show that if extrinsic factors maintain variation in behaviour, high levels of co-operation are stable. Our results highlight the importance of extrinsic variability in determining the outcome of evolutionary games.     

Cooperation and conflict in quorum-sensing bacterial populations

It has been suggested that bacterial cells communicate by releasing and sensing small diffusible signal molecules in a process commonly known as quorum sensing (QS). It is generally assumed that QS is used to coordinate cooperative behaviours at the population level. However, evolutionary theory predicts that individuals who communicate and cooperate can be exploited. Here we examine the social evolution of QS experimentally in the opportunistic pathogen Pseudomonas aeruginosa, and show that although QS can provide a benefit at the group level, exploitative individuals can avoid the cost of producing the QS signal or of performing the cooperative behaviour that is coordinated by QS, and can therefore spread. We also show that a solution to the problem of exploitation is kin selection, if interacting bacterial cells tend to be close relatives. These results show that the problem of exploitation, which has been the focus of considerable attention in animal communication, also arises in bacteria.     

SINR-balancing based cooperative spectrum sharing scheme for MIMO cognitive radio systems

In order to improve the system capacity of the primary user （PU） and secondary user （SU） of multiple-input-multiple-output （MIMO） cognitive radio （CR） system, a signal to interference plus noise ratio balancing （SINR-balancing） posed, in which PU leases a fraction of based cooperative spectrum sharing （CSS） scheme is proits transmission time to SU in exchange for the SU relaying the PU＇ s data cooperatively. The SINR-balancing based corresponding beamforming vectors are designed and time-division is also optimized for the proposed scheme. Simulation results show that compared to conventional opportunistic spectrum sharing （OSS） scheme, the proposed CSS scheme can effectively enhance the system performance of both PU and SU and provide an effective cooperation mechanism for PU and SU to determine whether to request cooperation.     

Monkeys reject unequal pay

During the evolution of cooperation it may have become critical for individuals to compare their own efforts and pay-offs with those of others. Negative reactions may occur when expectations are violated. One theory proposes that aversion to inequity can explain human cooperation within the bounds of the rational choice model, and may in fact be more inclusive than previous explanations. Although there exists substantial cultural variation in its particulars, this 'sense of fairness' is probably a human universal that has been shown to prevail in a wide variety of circumstances. However, we are not the only cooperative animals, hence inequity aversion may not be uniquely human. Many highly cooperative nonhuman species seem guided by a set of expectations about the outcome of cooperation and the division of resources. Here we demonstrate that a nonhuman primate, the brown capuchin monkey (Cebus apella), responds negatively to unequal reward distribution in exchanges with a human experimenter. Monkeys refused to participate if they witnessed a conspecific obtain a more attractive reward for equal effort, an effect amplified if the partner received such a reward without any effort at all. These reactions support an early evolutionary origin of inequity aversion.     

Reputation helps solve the 'tragedy of the commons'.

The problem of sustaining a public resource that everybody is free to overuse-the 'tragedy of the commons'-emerges in many social dilemmas, such as our inability to sustain the global climate. Public goods experiments, which are used to study this type of problem, usually confirm that the collective benefit will not be produced. Because individuals and countries often participate in several social games simultaneously, the interaction of these games may provide a sophisticated way by which to maintain the public resource. Indirect reciprocity, 'give and you shall receive', is built on reputation and can sustain a high level of cooperation, as shown by game theorists. Here we show, through alternating rounds of public goods and indirect reciprocity games, that the need to maintain reputation for indirect reciprocity maintains contributions to the public good at an unexpectedly high level. But if rounds of indirect reciprocation are not expected, then contributions to the public good drop quickly to zero. Alternating the games leads to higher profits for all players. As reputation may be a currency that is valid in many social games, our approach could be used to test social dilemmas for their solubility.     

A novel QoS supported cooperative diversity algorithm for IEEE 802.11 WLAN

A new cooperative diversity algorithm with quality of service （QoS） awareness for wireless local area network （WLAN） is proposed. According to this protocol, cooperation is invoked only if a direct transmission cannot meet user-specific QoS requirements. If cooperation is implemented, partner choice depends on who can achieve the minimum data rate required to have a successful transmission. The closed-form expressions for the system throughput and the power consumption are provided. Theoretical analysis and simulation results indicate that the QoS-aware cooperative scheme can significantly improve the system performance in terms of throughput and energy efficiency by avoiding negative effect of unconditional cooperation.     

认知无线电协作频谱感知混合加权算法

协作频谱感知的关键技术是降低控制信道开销,笔者提出了一种基于信噪比的加权协作检测算法,采用双门限混合加权判决,融合中心根据当前检测信道贡献大小,从而计算和分配相应的加权值,降低无效的传输.模拟结果表明：该算法不仅检测概率有明显的提高外,而且平均感知比特数下降,从而降低了公共信道的开销.     

复杂网络上博弈行为演化的合作激励

通过引入人类社会组织的激励机制,研究网络初始状态存在少量合作者时的囚徒困境博弈行为演化,着重探讨激励机制在网络结构作用条件下对群体合作行为产生的影响.首先从理论层面分析当方格网上存在一个合作者时,合作行为演化形成合作者聚集结构的激励条件;然后采用仿真实验方法研究方格网、规则小世界网络和异质小世界网络上随机分布少量合作者时,激励机制对合作行为演化的影响.研究发现,激励机制在不同的网络结构上总体表现出对合作行为演化的促进作用.但是在方格网和规则小世界网络上,由于节点度的同质化效应,针对合作行为的激励有时不一定会产生更好的合作状态,甚至可能起到负面的效果.     

小世界网络中的公共物品博弈

研究了小世界网络中的公共物品博弈,分析了噪声以及网络节点的异质性对博弈行为的影响．我们发现,网络的小世界特性以及网络中的噪声对博弈过程中合作的涌现起抑制作用,而网络节点的异质性则对博弈过程中合作的涌现起促进作用．我们用平均场理论对得到的数值模拟的结果进行了对比和分析．     

Spatial prisoner’s dilemma games with increasing size of the interaction neighborhood on regular lattices

We studied the evolution of cooperation in the prisoner’s dilemma game on a square lattice where the size of the interaction neighborhood is considered. Firstly, the effects of noise and the cost-to-benefit ratio on the maintenance of cooperation were investigated. The results indicate that the cooperation frequency depends on the noise and cost-to-benefit ratio: cooperation reaches a climax as noise increases, but it monotonously decreases and even vanishes with the ratio increasing. Furthermore, we investigated how the size of the interaction neighborhood affects the emergence of cooperation in detail. Our study demonstrates that cooperation is remarkably enhanced by an increase in the size of the interaction neighborhood. However, cooperation died out when the size of the interaction neighborhood became too large since the system was similar to the mean-field system. On this basis, a cluster-forming mechanism acting among cooperators was also explored, and it showed that the moderate range of the neighborhood size is beneficial for forming larger cooperative clusters. Finally, large-scale Monte Carlo simulations were carried out to visualize and interpret these phenomena explicitly.     

Image scoring and cooperation in a cleaner fish mutualism

Humans are highly social animals and often help unrelated individuals that may never reciprocate the altruist's favour. This apparent evolutionary puzzle may be explained by the altruist's gain in social image: image-scoring bystanders, also known as eavesdroppers, notice the altruistic act and therefore are more likely to help the altruist in the future. Such complex indirect reciprocity based on altruistic acts may evolve only after simple indirect reciprocity has been established, which requires two steps. First, image scoring evolves when bystanders gain personal benefits from information gathered, for example, by finding cooperative partners. Second, altruistic behaviour in the presence of such bystanders may evolve if altruists benefit from access to the bystanders. Here, we provide experimental evidence for both of the requirements in a cleaning mutualism involving the cleaner fish Labroides dimidiatus. These cleaners may cooperate and remove ectoparasites from clients or they may cheat by feeding on client mucus. As mucus may be preferred over typical client ectoparasites, clients must make cleaners feed against their preference to obtain a cooperative service. We found that eavesdropping clients spent more time next to 'cooperative' than 'unknown cooperative level' cleaners, which shows that clients engage in image-scoring behaviour. Furthermore, trained cleaners learned to feed more cooperatively when in an 'image-scoring' than in a 'non-image-scoring' situation.     

论广西与东盟诸国合作模式的转换

通过对广西近些年来与东盟各国合作关系的考察,反思在合作观念和合作实践方面存在的偏差和问题,认为梯度差异和互补性只是广西与东盟合作的必要条件而不是充分条件;经济合作只是广西与东盟合作的一个重要方面,还应该包括政治、社会、安全、体育、卫生等方面的合作;政企合作也只是广西与东盟合作的模式之一,还包括政府与政府、市场主体与市场主体、非营利组织与非营利组织等之间的合作。     

Spatial structure often inhibits the evolution of cooperation in the snowdrift game

Understanding the emergence of cooperation is a fundamental problem in evolutionary biology. Evolutionary game theory has become a powerful framework with which to investigate this problem. Two simple games have attracted most attention in theoretical and experimental studies: the Prisoner's Dilemma and the snowdrift game (also known as the hawk-dove or chicken game). In the Prisoner's Dilemma, the non-cooperative state is evolutionarily stable, which has inspired numerous investigations of suitable extensions that enable cooperative behaviour to persist. In particular, on the basis of spatial extensions of the Prisoner's Dilemma, it is widely accepted that spatial structure promotes the evolution of cooperation. Here we show that no such general predictions can be made for the effects of spatial structure in the snowdrift game. In unstructured snowdrift games, intermediate levels of cooperation persist. Unexpectedly, spatial structure reduces the proportion of cooperators for a wide range of parameters. In particular, spatial structure eliminates cooperation if the cost-to-benefit ratio of cooperation is high. Our results caution against the common belief that spatial structure is necessarily beneficial for cooperative behaviour.     

论上海合作组织的合作愿景

上海合作组织有美好的合作愿景,这是由中俄两国共同推动组织发展的统一意志、中亚国家的合作态度、观察员国和对话伙伴国的积极支持等因素决定的。共同的反对霸权主义和强权政治的任务使它们日益结合成了一个命运共同体。维护全球战略平衡与稳定、推动建立国际政治经济新秩序是上合组织首要的、核心的职能和任务,因而政治合作是上合组织存在的灵魂,经济贸易合作和人文合作则为政治合作奠定坚实根基。     

活跃连接下两个水平上的合作进化

本文研究活跃链接下两个水平（即组选择）的合作进化动态．给出了弱选择下策略的固定概率及策略更有优势的条件．然后以囚徒困境为例讨论了促进合作行为进化的条件，即如果支付收益比与总体内组数、组容量及各策略间接触率满足一定的不等式，则有利于合作行为的进化．最后，本文对模型进行了数值模拟，结果表明，本文模型可以有效促进合作行为的进化．