The Paradox of Altruism
Altruism — helping others at a cost to oneself — seems to contradict natural selection. If selfish individuals outcompete altruists, how can genes for self-sacrifice persist? W.D. Hamilton resolved this paradox in 1964 with a simple but profound insight: natural selection acts on genes, not individuals. A gene for altruism can spread if it causes its bearer to help relatives who carry copies of the same gene.
Hamilton's Rule: rB > C
The elegant inequality rB > C captures the entire logic of kin selection. The relatedness coefficient r measures the probability that actor and recipient share a gene by common descent. Full siblings share r = 0.5; half-siblings r = 0.25; cousins r = 0.125. The rule says: sacrifice yourself only if the benefit B to relatives, weighted by their relatedness r, exceeds the cost C to you.
Inclusive Fitness in Action
Kin selection explains a vast array of biological phenomena. Worker bees sacrifice their reproduction to help their mother queen — haplodiploid genetics makes sisters more related (r = 0.75) than mothers to daughters (r = 0.5). Ground squirrels give alarm calls that endanger themselves but warn nearby kin. Even plants distribute resources preferentially to closely related neighbors.
The Reach of Hamilton's Insight
Hamilton's framework extends beyond obvious altruism. Parent-offspring conflict (Trivers 1974) arises because parents and offspring have different optimal investment levels (r = 0.5, not 1.0). Sibling rivalry reflects competition among relatives with r = 0.5. Genomic imprinting — where genes are expressed differently depending on parent of origin — creates an intragenomic battleground over resource allocation. Kin selection theory pervades modern evolutionary biology.