What Does “Relatedness” Mean in Hamilton’s Rule?

According to Hamilton’s rule, the coefficient of relatedness (r) is the genetic relatedness between the individual performing the altruistic act (the actor) and the recipient of the act. This relatedness quantifies the probability that a gene present in the actor is also present in the recipient due to common descent.

For example, if a meerkat gives an alarm call that warns its siblings, the relatedness (r) is measured between the calling meerkat (the actor) and its siblings (the recipients). If the benefit to the siblings, weighted by this relatedness, exceeds the cost to the caller, the behavior can evolve through natural selection.

Why Not Offspring or Population?

• Offspring of the recipient: Hamilton’s rule is not directly concerned with the recipient’s offspring, but with the recipient themselves.

• Offspring of the actor: The actor’s own reproductive success is part of their direct fitness, not the indirect fitness that Hamilton’s rule addresses.

• Member of the population: The coefficient of relatedness is not generally measured between the actor and any random population member, but specifically between the actor and the recipient of the altruistic act.

The Correct Relationship

Therefore, the correct answer is:
(2) individual performing the altruistic act and the recipient

This relationship is at the heart of Hamilton’s rule and the evolutionary logic of kin selection.

Correct answer: (2) individual performing the altruistic act and the recipient