Definition
Inclusive fitness is the measure of an organism's total contribution to the next generation's gene pool, counting both:
- Direct fitness — the organism's own reproductive output, and
- Indirect fitness — the reproductive output of its relatives, weighted by genetic relatedness r.
Introduced by W. D. Hamilton in 1964, inclusive fitness generalizes Darwinian fitness from a measure of individual reproductive success to a measure of gene-level reproductive success. It is the technical reframing that makes kin selection rigorous.
Why it matters
How it works
Imagine a gene that disposes its bearer to help a relative. The cost to the bearer is C reproductive units; the benefit to the relative is B reproductive units. The relative shares the gene with probability r (the relatedness coefficient: ½ for parent-child or full siblings, ¼ for half-siblings or niece/nephew, ⅛ for first cousins).
The gene's expected gain through the indirect route is rB (the relative's gain × the probability the relative carries the gene). The gene's expected loss is C (the bearer's cost). Net change in copies: rB − C. If positive, the gene spreads.
Adding C (the direct cost) back into the equation gives the inclusive measure: the gene's full impact on the next generation's gene pool. Selection maximizes inclusive fitness, not direct fitness.