Eusociality is a cooperative living arrangement in which members of a species give up their own chances of reproduction to help raise the offspring of others. Traditionally, researchers have thought that eusociality arises through “kin selection theory.” Members of the group who give up their own reproduction to help others aren’t really altruistic; by helping their kin—who are genetically similar—they are actually favoring their own genes indirectly. With evolutionary biologist E.O. Wilson and mathematical biologist Corina Tarnita, Martin has come up with an alternative explanation: They argue that natural selection alone can explain the evolution of eusocial behavior through a number of steps. If a species follows these steps—its members stick together and develop the traits and genes that cause them to cooperate—eusociality is the result.
Mathematical models show that natural selection, on its own, opposes cooperation. Non-cooperators will do better than cooperators and wipe them out. So natural selection needs help to favor cooperation—mechanisms that make sure we get more from cooperating than being selfish. There are five of these mechanisms. Direct reciprocity is based on repeated encounters between the same two individuals: I help you and you help me. Indirect reciprocity is based on reputation: I help you and somebody else helps me. Spatial selection means that clusters of cooperators can prevail: Neighbors help each other. Group selection occurs if there is competition between groups: The members of a group help each other. Kin selection is based on interactions between close genetic relatives: Brothers help each other, for example.
Because we spend most of our lives in a relatively small population in which we interact with the same people again and again, we continually monitor and interpret how people act. How we decide to behave toward others depends on both what they have previously done to us and how they have treated others. When people are asked if they want to donate money to another person, for example, research finds that people base their decision on what that person has done in the past. Generous people are more likely to receive donations. When we are deciding how to act, we also often subconsciously take into account the possible consequences for our own reputation.