In this blog post, we’ll examine the relationship between altruistic behavior observed in groups and natural selection, and explore the core concepts and limitations of the group selection hypothesis—which posits that individual selection and group selection can act in opposite directions.
One of the greatest joys of my school days was playing soccer with friends during lunch or P.E. class. All it took was a single soccer ball to shake off the stress of schoolwork, even if only for a moment. While playing soccer, we went through a cycle of wins and losses, but looking back on the games we won, I realize that in many cases, we achieved victory by working in perfect harmony with my friends to neutralize the opposing team’s offense and defense. Of course, there are times when a single hero leads the team to victory, but we know that, in most cases, the chances of winning are higher when there is good teamwork. This isn’t limited to soccer games. In video games, too, we often win when we trust and help one another, and even during cleanup time, we can finish faster by working together. In this way, altruistic behavior within a group increases efficiency and, furthermore, helps secure an advantage over other groups in competition.
This fact is not limited to modern times. Ever since humanity began forming groups in the distant past, the existence of altruistic individuals has brought great benefits to the group and likely increased the survival chances of its members. In other words, the existence of altruistic individuals may have played a significant role in the history of human evolution. Evolutionary biologists have studied the relationship between groups and altruistic individuals, and one of the evolutionary perspectives that emerged from this research is the group selection hypothesis.
To understand the group selection hypothesis, one must first understand what natural selection is. Natural selection is the process by which individuals possessing traits that are relatively advantageous for survival and reproduction in a specific environment contribute more to the next generation, thereby increasing the prevalence of those traits within the group. This means that, over time, the population may come to consist of more individuals with traits better suited to the environment. However, we can generally consider that selfish behavior may be more advantageous for an individual’s survival and reproduction than altruistic behavior. Just as a person who spends their entire salary on themselves is likely to have a higher material quality of life than someone who donates a portion of it to society, even if they earn the same annual salary. Even in ancient times, it is likely that individuals who prioritized using the resources they acquired for themselves had an advantage in terms of survival and reproduction compared to those who shared food with others. From this perspective, behaviors that maximize individual benefit are more likely to be selected. From the perspective of individual selection, altruistic behavior often imposes a cost on the individual; therefore, altruistic individuals may be at a disadvantage when competing with selfish individuals, and such tendencies may be reduced by natural selection. Therefore, considering the long history of human existence, the fact that a significant number of people exhibit altruistic tendencies in modern times can be seen as an evolutionary puzzle.
The “group selection hypothesis” is one of the theoretical perspectives that explains the existence of such altruistic individuals. According to the group selection hypothesis, in competition between groups, the more altruistic individuals a group has, the more likely it is to gain a competitive advantage. This suggests that a group with many altruistic individuals—which gains an advantage in intergroup competition—may displace groups lacking such individuals from the ecosystem, or that as other groups learn from the behavior of successful groups, the proportion of individuals exhibiting cooperative behavior within the population as a whole may increase. In recent evolutionary biology, this discussion is sometimes viewed from the perspective of multilevel selection, where individual-level selection and group-level selection act in concert. In fact, a growing body of research suggests that if differences between groups are sufficiently large and intergroup competition is intense, group-level selection can contribute to the spread of cooperative behavior. Anyone who plays multiplayer team-based games these days has likely already experienced the importance of altruistic behavior firsthand. In team fights like “Han-ta,” a team composed of players who trust their teammates, boldly charge into enemy territory, and—even when survival seems unlikely—refuse to flee and continue dealing damage—that is, chipping away at the opposing team’s health—is more likely to win than a team made up of players who flee to save their own skins. Therefore, in competition between groups, a group with many “altruistic individuals” who are willing to sacrifice themselves for others is more likely to win, and at the group level, it can be argued that such groups possess traits advantageous for survival and competition. If this is true in the virtual world, what about the real world? It’s likely that similar situations occurred in countless battles throughout history. One might reason that a military unit with many soldiers willing to sacrifice their lives for their tribe or nation by charging into enemy lines is more likely to defeat a unit composed of soldiers who abandon their comrades and are too busy fleeing to save themselves. However, the extent to which competition between groups actually played a significant role in explaining the evolution of cooperation remains a subject of debate, and one cannot conclude that group selection was at work simply because a group had many cooperative individuals.
Let’s delve a little deeper into the topic of war. We’ve discussed that the side with many altruistic soldiers who risk their lives to fight the enemy is more likely to win a battle. In other words, a group with many altruistic individuals can gain an advantage in intergroup competition. From the group’s perspective, having many altruistic individuals can be an advantageous strategy. This, however, is viewed from the group’s perspective. But what about from the individual’s perspective? Can a soldier who risks his life to secure victory actually survive? Naturally, his chances of survival would decrease. Fleeing might actually be a more advantageous strategy when considering only the individual’s survival. Thus, individual selection and group selection can work in opposite directions. In the process of individual selection, behaviors that are advantageous to the individual—even if they are uncooperative toward the group—may be selected; however, in the process of group selection, a group with many cooperative members may gain a competitive advantage. Therefore, for group-level selection to actually take effect, differences between groups must be maintained, and the effects of intergroup competition must sufficiently outweigh the effects of selection occurring within individual groups.
As mentioned above, individual selection and group selection can operate in opposite directions. Therefore, we can see that the relative strength of these two forms of selection will have a significant impact on the proportion of individuals exhibiting each tendency within the entire group. To support the validity of the group selection hypothesis, one must demonstrate that conditions exist under which the effects of group selection can outweigh those of individual selection—a task that is not easy. Many scholars have argued that when selection acting within a group is strong, altruistic behavior beneficial to the group may become disadvantageous. However, there are several reasons why group selection likely had ample opportunity to act. Considering ancient times, competition among early human groups may have served as a significant selective pressure, and intergroup conflict and high mortality rates may also have created differences at the group level. Furthermore, in ancient times, a high proportion of protein and fat intake came from animal-based foods obtained through hunting, and cooperation among group members would have been crucial for hunting. Finally, modern humans are a species that has endured harsh environments such as the Ice Age; to survive these conditions, cooperative behaviors—such as finding safe shelter or protecting group members from animal attacks—would likely have been necessary. However, it is not easy to directly verify to what extent these prehistoric environments and intergroup competition actually selected for cooperative tendencies in humans. While recent studies have suggested that cultural group selection may explain the evolution of human cooperation, debate continues regarding the nature and intensity of intergroup competition, as well as how long intergroup differences persisted.
So far, we have examined the group selection hypothesis and its limitations. Given that groups with more altruistic individuals can gain an advantage in intergroup competition, the group selection hypothesis offers an important perspective for explaining the emergence of altruistic humans and the spread of cooperation. However, because individual selection and group selection can act in opposite directions, it is difficult to explain the survival and spread of altruistic individuals through group selection alone. In modern evolutionary biology, this is often treated not as a matter of choosing between individual selection and group selection, but rather as a problem of multilevel selection acting simultaneously within individuals and between groups. In particular, under conditions where differences between groups are sufficiently large, intergroup competition is strong, and the disadvantages faced by altruistic individuals within a group can be offset by group-level benefits, group-level selection can contribute to the evolution of cooperative behavior. Therefore, rather than being a universal theory that explains altruistic behavior, group selection can be regarded as an important theoretical framework for understanding how individual-level and group-level selection interact. In human society, since group norms, institutions, and cultural learning can influence individual behavior and differences between groups, the perspective of group selection—or multilevel selection—remains a meaningful research topic for understanding human cooperation and altruistic behavior.
