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Kin Selection

Kin selection refers to the evolutionary process leading to adaptations that promote altruism among close genetic relatives. Also known as Inclusive Fitness Theory, Kin Selection Theory was first described by William Hamilton in 1964 and is perhaps the most significant addition to Darwin's theory of evolution by natural selection in the 20th century. At the time of Hamilton's publication, altruism had been a biological mystery; there was no cogent account for why evolution would select for altruistic behaviors that reduced one's own chances of surviving and reproducing and enhanced the survival and reproduction of another. After all, natural selection was thought to produce solely selfish behaviors—a “nature red in tooth and claw.” Hamilton's elegant theory provided the missing logic for how altruism could have evolved. This entry discusses the logic of kin selection and provides examples of the kinds of questions Kin Selection Theory can address.

The Logic of Kin Selection

The key to understanding kin selection is to take a gene's-eye view. A gene, unlike an individual, can propagate in two different ways. The first is by promoting the survival and reproduction of the body in which it resides. The second is by promoting the survival and reproduction of other bodies that have a high probability of possessing an identical copy. Who is likely to share a copy of the same genes? By virtue of sharing common ancestors, close biological relatives have a greater than average chance of sharing genes. The more closely related kin are to one another, the greater the likelihood they will share genes. For instance, nuclear family members (mother, father, children, and siblings) on average have a probability of .5 of sharing a particular gene in common. The probability of sharing a particular gene in common with a grandparent, niece, nephew, aunt, uncle, or half sibling drops to .25; a first cousin drops to .125, and so on. This probability describes the degree of relatedness between two individuals and is a crucial component of Kin Selection Theory. An example is provided for how to compute degree of relatedness at the end of this entry.

Hamilton proposed a set of mathematical equations that captures the rules evolution might have approximated to shape a system producing kin-directed altruism. In its most basic form, kin selection can be represented by the equation riCi < rjBj. This states that selection will tend to favor altruistic motivations when the costs associated with individual i performing an altruistic act (Ci) weighted by individual i's degree of relatedness to himself (ri) are less than the benefits bestowed on recipient j (Bj) discounted by the i's degree of relat-edness to j (rj). Since ri equals 1 (people have a probability of 1 of having the same genes as themselves), the equation is typically written C < rB, where it is understood that the person performing the altruistic deed is oneself and another person is the beneficiary.

Questions Addressed by Hamilton's Equation

Hamilton's equation is a powerful tool for investigating when it pays to behave altruistically (or selfishly) toward another and when one should want others to behave altruistically (or selfishly) toward oneself or related others. It also provides a means of examining conflicts of interest. For instance, since Bart is more closely related to himself, he may want to be selfish and not share his Butterfinger with his sister Lisa (maybe just a crumb), but his mother Marge likely sees the world differently and would want Bart to share right down the middle since she is equally related to Bart and Lisa.

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