Why do living things behave altruistically? The kin selection hypothesis explains this in terms of genes, but it doesn’t explain all altruism. Let’s explore the evolutionary implications of altruistic behavior through a variety of hypotheses.
Charles Darwin’s theory of natural selection, published in 1859 in The Origin of Species, is the foundation of modern evolutionary theory. When competition for survival occurs in a given environment, only individuals with traits that favor adaptation to the environment will survive. This is called natural selection, and the surviving individuals leave behind offspring, and the process repeats itself, leading to evolution. Common sense would suggest that selfish individuals would have an advantage in the race to survive and leave the most offspring, because they’re more likely to survive and pass on their genes to the next generation if they’re selfishly looking out for their own survival without thinking about others.
But if you think about it, organisms don’t always act selfishly – think of a parent who would sacrifice their life for their child. Why do organisms go against their instinct to survive in a survival competition and act altruistically? If they were to die, they would be at an evolutionary disadvantage, as they would be less likely to pass on their genes to their offspring. Let’s take a look at one of the many explanations for the emergence of altruism: the kin selection hypothesis.
The kin selection hypothesis explains that individuals who perform altruistic acts are actually acting out of selfish motivations to spread their genes. Put yourself in the shoes of a gene. A gene doesn’t have to act selfishly to reproduce itself widely: if it can sacrifice itself for its own children, nieces, nephews, uncles, or other related individuals to keep them safe, it can pass on its own genes, which it shares in part with those related individuals, to the next generation. If there’s a more favorable way to spread itself, the gene might choose to abandon the individuals that contain it.
The measure of how many genes you share with an individual you’re related to is called consanguinity. To better understand the concept of consanguinity, let’s compare genes to marbles and organisms to bags, each containing two marbles. When genes are passed on to offspring through reproduction, mom and dad contribute one marble each, giving the offspring a total of two marbles. Since the offspring received half of the mother’s genes, we can say that the mother-child relationship is 0.5. The same is true for the father. But what about the siblings? Let’s call the marbles from Dad’s side A and B, and the marbles from Mom’s side a and b. Let’s say the first born child inherited one of each, resulting in Aa. The next born child will be Aa (1/4 chance) if they have exactly the same genes as their brother (1 gene share), Ab and Ba (2/4 chance) if they share half their genes with their brother (1/2 gene share), and Bb (1/4 chance) if they have completely different genes (0 gene share). Calculating the probability in each case and adding them together calculates the degree of consanguinity. So, the degree of consanguinity between siblings is 1/2. (Of course, in real life, individuals have more genes and different things happen during the inheritance process, but we’ve made things simple for the sake of understanding.)
Let’s use the concept of inbreeding to understand how it is genetically more advantageous to sacrifice for individuals who are related to you. Let’s say you have a family consisting of mom, dad, older brother, and younger brother. Your brother and the other family members are all related by 0.5. Your brother could act selfishly and protect his entire gene pool of 1, but if he can save the other three by sacrificing himself, he can also protect 0.5 × 3 = 1.5 of his own genes. Thus, an organism can act altruistically by sacrificing itself to preserve its own genes and protect those of its blood relatives.
One of the best examples of the kin selection hypothesis is bees. In bee colonies, worker bees dedicate their lives to caring for the eggs laid by the queen, and will defend them with their lives if an intruder appears. While this may seem like an altruistic act at first glance, the queen’s eggs that the worker bees protect are actually related to them by blood, as close as 0.5. The worker bee was acting to protect her own genes. Before we explain why the queen’s egg is 0.5 related to the worker bees, let’s briefly explain the kinship system in bee colonies. Some of the reproductive cells that the queen makes are fertilized by sperm from male bees and become worker bees or queen bees. To calculate the closeness of worker bees and queen bees laid by the same queen, we first calculate that the probability of receiving the same genes from the same father male bee is 1 and the probability of receiving the same genes from the queen is 1/2, so the closeness is 0.75 = (0.5 × 1 + 0.5 × 0.5). If we calculate the closeness of the worker bees produced by a queen to the male bees from which the queen’s gametes grew, the closeness is 0.25 = (0.5 × 0.5) because the probability of receiving the same gene from the queen is 1/2. Therefore, if we calculate the closeness of the worker bees to the queen’s eggs, we get 0.5 = ( 0.5 × 0.75 + 0.5 × 0.25). We can interpret the ostensibly altruistic behavior of the worker bees as being motivated by a selfish motive to protect their own genes.
In summary, while selfish individuals often have an evolutionary advantage in survival competitions, many organisms actually engage in altruistic behavior. Of the many hypotheses to explain this, the kin selection hypothesis explains altruistic behavior among related individuals from a genetic perspective better than any other. Altruistic behaviors, such as sacrificing for others, are actually about protecting one’s own genes. The measure of how closely related individuals share genes is called inbreeding. The altruistic behavior of worker bees, a classic example of the kin selection hypothesis, can be interpreted as passing on their genes to the next generation more effectively by protecting the eggs of their queen bee, who is 0.5 inbred to them. While the kin selection hypothesis is a very effective explanation for altruistic behavior among closely related individuals, it does not explain all altruistic behavior in nature. For example, in a population of meerkats, there are immigrants who are not related to any of the meerkats, and when scientists looked at the number of times each meerkat watched, they found no difference in the number of times the meerkats watched between their kin and the immigrant meerkats. We often see altruistic behaviors in meerkats and humans that are not related to them, even though they are doing things that are a threat to their own lives. While the kin selection hypothesis provides a good explanation for the motivation of altruistic behavior among kin, it is limited in that it does not provide a logical basis for altruistic behavior among non-kin.
There are a number of other hypotheses that address the limitations of the kin selection hypothesis, such as the group selection hypothesis and the eusocial species hypothesis, and each of these hypotheses may be able to better explain the emergence of altruism by capitalizing on their strengths and compensating for each other’s limitations. For example, the group selection hypothesis states that when an individual’s altruistic behavior contributes to the survival and prosperity of the group, the group as a whole will be more successful in its evolution. In addition, the eusociality hypothesis explains that altruism allows individuals with similar behaviors to benefit more from cooperation. These different hypotheses contribute to explaining the evolutionary reasons for altruistic behavior.
These hypotheses can also be applied to human societies. Altruistic behavior in humans is not limited to kinship, but is manifested more broadly through social bonds and cooperation. This suggests that humans are undergoing a more complex and multi-layered evolutionary process that goes beyond mere survival and involves social relationships and cultural development. In this respect, human altruistic behavior should be understood alongside a variety of evolutionary theories that complement Darwin’s theory of natural selection.
Finally, the study of altruistic behavior is more than just an academic curiosity; it can have practical implications for our society and lives. By understanding the roots of altruistic behavior, we may be able to gain the insights we need to build a more cooperative and harmonious society.
