The Puzzle of Altruism
Altruism is any behavior that increases another's fitness while decreasing the actor's own. Natural selection should favor selfishness, making this a major puzzle.
Hamilton's Rule resolves this via kin selection. Altruism is favored when , where:
| Symbol | Meaning |
|---|---|
| r | Genetic relatedness |
| b | Fitness benefit to recipient |
| c | Fitness cost to actor |
Helping a relative spreads your own genes indirectly through shared ancestry.
Common pitfall: Altruism in biology requires no conscious kindness. It is defined purely by fitness consequences, regardless of intent.
Inclusive Fitness & Reciprocity
Inclusive fitness is an individual's direct reproductive success plus indirect success achieved by helping relatives reproduce, weighted by . A sterile worker bee achieves high inclusive fitness by raising related sisters.
Beyond kin, reciprocal altruism explains cooperation among unrelated individuals. Help is favored if the cost is low, the benefit is high, and the favor is likely to be returned.
| Type | Key Driver | Example |
|---|---|---|
| Kin Selection | Genetic relatedness () | Social bees raising siblings |
| Reciprocal Altruism | Expected future return | Vampire bats sharing blood |