Kin selection
Kin selection is a concept in evolutionary biology that explains how altruistic behaviors—actions that benefit others at a cost to oneself—can evolve among organisms. This theory extends the principles of natural selection by emphasizing the importance of genetic relationships among individuals. Proposed by evolutionary biologist William D. Hamilton in the 1960s, kin selection is based on the idea of inclusive fitness, which consists of direct fitness (an organism's own reproductive success) and indirect fitness (the success of relatives who share some of the same genes).
Hamilton introduced a mathematical rule to predict when an organism would sacrifice its own reproductive opportunities to assist its relatives. The degree of relatedness, quantified as the coefficient of relatedness, plays a crucial role in this decision-making process. For instance, individuals are more likely to help their siblings, who share 50% of their genes, than their cousins, who share only 12.5%. Examples of kin selection can be seen in various species, such as birds that forgo reproduction to assist family members or squirrels that emit alarm calls to warn relatives of danger.
While the concept has garnered significant support, it remains a topic of debate among scientists, with some suggesting that behaviors attributed to kin selection might also arise from direct natural selection. Overall, kin selection provides a framework for understanding the evolution of cooperative behaviors among closely related individuals.
Kin selection
An extension of the theory of natural selection, kin selection occurs when an organism engages in altruistic (selfless) behavior that improves the survival and reproduction of its relatives. Kin selection rests on the theory of inclusive fitness, which has two components: direct fitness and indirect fitness. Evolutionary biologist William D. Hamilton first proposed the theory of inclusive fitness in the 1960s to explain why some organisms have evolved to help others of the same species. Hamilton developed a mathematical rule that underpins the theory. This rule takes into account the cost to the helper, the benefit to the helped, and the degree of relatedness between the two.
Background
In 1859, evolutionary biologist Charles Darwin published On the Origin of Species, in which he introduced his theories on evolution and natural selection. Natural selection is often explained very basically as "survival of the fittest." However, natural selection depends not only on survival but also on reproduction.
In any ecosystem, resources are limited. Food, water, living space, and mates are all available in fixed quantities. Organisms have to compete with one another for these resources. Organisms with traits that allow them to adapt to their surroundings and gain superiority over their peers are more likely to survive than organisms lacking these traits. These superior survival traits are one aspect of natural selection.
The second aspect of natural selection is reproduction. If the organisms that are able to adapt and survive are also able to reproduce, they will pass their superior traits to their offspring. The fittest of these offspring will survive and reproduce, thereby continuing to pass these superior traits to subsequent generations. So, natural selection is more than just survival of the fittest. It involves selection for or against specific genetic traits that promote or hinder an organism's ability to survive and reproduce. Traits that increase an organism's abilities to survive and reproduce are said to be "selected for." Traits that hinder these abilities are said to be "selected against."
One problem with the theory of natural selection is that it does not explain why some organisms engage in altruistic, or self-sacrificing, behaviors that reduce their own odds of survival and reproduction just to aid in the survival and reproduction of others. The concept of kin selection attempts to answer this question.
Overview
If natural selection depends on the survival of the fittest, then logic dictates that altruistic behaviors would be selected against. Organisms that sacrifice their own survival and reproductive abilities to help others would be less likely to survive and pass their traits to subsequent generations. Based solely on the theory of natural selection then, the trait responsible for these altruistic behaviors would disappear over time. Yet, members of many species of birds, animals, and insects continue to demonstrate altruism, suggesting that this trait, at least in some cases, must be selected for.
In 1964, evolutionary biologist William D. Hamilton proposed the idea of inclusive fitness. Inclusive fitness suggests that an organism can guarantee the proliferation, or continued spread, of its genes in two ways: direct fitness and indirect fitness. Direct fitness is the process by which an organism passes its genes directly to its own offspring through reproduction. Indirect fitness is the process by which an organism helps relatives survive and reproduce. The helping organism and its relatives likely share some of the same genes. By ensuring the survival and reproduction of its relatives, the helping organism increases the likelihood that some of the "helper" genes will be passed to subsequent generations. In this way, the traits of helping organisms—which have a positive effect on their relatives—are selected for. This process is called kin selection.
According to Hamilton, the degree of relatedness between organisms is important to kin selection. The amount of help an organism provides to relatives depends on how closely the organism is related to them. The more genes the helper and its relatives share, the higher the degree of relatedness between them. This value is represented as a ratio called the coefficient of relatedness. The coefficient of relatedness for siblings is 1/2 because the probability that siblings have a certain gene is 0.5. The coefficient of relatedness for cousins is 1/8 because the probability that cousins have a certain gene in common is 0.125. Therefore, a helper organism is four times more likely to help a sibling than a cousin.
Using the coefficient of relatedness, Hamilton developed a mathematical rule to predict the likelihood of an organism choosing to sacrifice its own chance to survive and reproduce to help its relative. The rule indicates that the helper will choose to assist only if the benefit the helper provides to its relative is greater than the cost to the helper of not reproducing. Hamilton's rule is represented as follows: B > C/r, where B is the benefit that the helper's relative receives, C is the cost that the helper incurs for helping, and r is the coefficient of relatedness.
Kin selection presents itself in different ways. For example, some bird species, such as Florida scrub jays, skip finding a mate and reproducing to help other members of their flock during breeding season. They may assist breeding family members by collecting food or defending the nest from predators. Alarm calls are another form of altruistic behavior that stems from kin selection. When a squirrel or a prairie dog lets out an alarm call to alert its kin to the presence of a predator, it calls attention to itself and puts itself in greater danger of being attacked. This self-sacrificing action helps protect the survival of the squirrel's or prairie dog's relatives. Members of ant, bee, and wasp species are also known to engage in certain altruistic behaviors that stem from kin selection.
Not everyone in the scientific community accepts inclusive fitness theory and the concept of kin selection. For example, in 2010, scientists at Harvard University in Cambridge, Massachusetts, used mathematical analysis to show that behaviors often attributed to kin selection could actually evolve as a result of natural selection. Although the analysis received some support, other scientists have argued that it is incorrect or fails to explain certain phenomena.
In 2021, a study published in Botanical Scientists alleged that kin selection could be used to explain certain phenomena in bacteria. Scientists argued that kin selection effectively explained observed patterns of cooperation across several microbial species. These advancements could lead to breakthroughs in microbial community management, such as engineering beneficial to microbial communities or reducing the population of harmful microbial communities.
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