Refer to the passage: Among the persistent puzzles in evolutionary biology is the existence of cooperation among unrelated individuals — a pattern that, at first glance, seems incompatible with the relentless logic of natural selection. If natural selection rewards organisms that maximize their own reproductive success, why do bees sacrifice themselves defending hives, why do vampire bats regurgitate food for unrelated companions, and why do humans pay taxes to support strangers? Several mechanisms have been proposed. Kin selection explains some cases: by helping relatives who share our genes, we indirectly propagate our own genetic material. Reciprocal altruism accounts for others: cooperation among unrelated individuals can be stable when interactions are repeated and defectors can be punished or avoided. Group selection, once dismissed, has been partially rehabilitated under more careful formulations involving differential survival of cooperative groups. Most intriguingly, in humans, cultural evolution may amplify and stabilize cooperative norms in ways that genetic evolution alone could not, suggesting that what makes our species peculiar is not the absence of self-interest but the elaborate institutional and cultural scaffolding that channels it. Kin selection explains cooperation primarily by appealing to: