Cooperation among creatures can guarantee their life and their survival . According to this fact , one can explain the formation of complex biological structures . Gathering together , the single cells can produce more complete and more resistant creatures . Charles Darwin was among the first scientists who introduced a theory for evolution by means of natural selection . His theory has three basic facts : Number of off springs are always greater than the number of their parents , the species' population is constant , and there are always small differences between inheritable characteristics of parents and children; therefore , it can be inferred that the surviving children are those who can better adapted to the environment . This phenomenon is known as natural selection . At this point understanding of cooperation formatio among individuals of one particular species , and then among different species , will be very important as natural selection opposes it . There are always differences , advantages and weaknesses even among individuals of a same species; so why stronger individuals help the weaker to survive? Cooperation is still distinguished among species for years; however , accurate explanation for the process of cooperation needs more effort . In 1972 Robert Trivers , an evolutionary biologist and mathematician , mentioned a simple model describing interaction of two independent entity , which appeared to be helpful in understanding of this altruistic behavior . Although it is beneficial for both entities to cooperate in the mentioned interaction , both will eventually decide to defect at the end . The mentioned interaction is actually a famous model in game theory introduced by two mathematician , Merrill Flood and Melvin Dresher , a basic model for studying interactions and cooperation in game theory . After that a mathematician named Albert Tucker formulated it and gave it the name ``prisoner's dilemma" . All of the interactions of the same type are known as ``games'' . There are a great variety of them , all of them are studied in game theory . Logic is the foundation of game theory; however , if game theory is used for describing evolution without considering logic , we use the ``evolutionary game theory" , which is concluding the result of a game as a function of population of individuals entering the game as specific species . There are a variety of methods for achieving cooperation in games . One of these methods use a limited resource . In this model a number will be given to each player , interpreted as the player's resource . This resource distributes among players as they interact , or increases because of an environmental resource which benefits all of the players . Whenever this resource reaches a specific threshold , the owner of the resource add an offspring to the population and share the resource with the offspring . Take into account this resource and death , it has been verified that cooperation can survive . A more complicated way of game is done on a graph , that is having nodes of the graph as players . In this case the only players who are connected through an edge can interact . The theory studying this type of games is known as evolutionary graph theory . In this research limited resource model is applied to a specific graph , scale-free graph , in order to show the function of a graph in intensifying the final result of the game.