inclusive fitness
Summary
Inclusive fitness is an individual's genetic success counted through its own offspring plus the extra offspring of relatives it helps, weighted by relatedness. It explains how altruism toward kin can evolve even when helping costs the helper reproductive success.
Detail
Meaning: Evolutionary fitness is reproductive success, the copies of one's alleles passed on. Inclusive fitness (W. D. Hamilton) adds direct fitness, from one's own offspring, to indirect fitness, from the extra offspring relatives produce because of one's help, each weighted by relatedness. Selection can therefore favor altruism toward kin (kin selection), since relatives carry copies of the same alleles by common descent. On average, r is 0.5 for parent-offspring and full siblings, 0.25 for half-siblings and grandparents, and 0.125 for first cousins.
Equation: rB > C, where r = coefficient of relatedness (unitless, 0 to 1), B = reproductive benefit to the recipient, and C = reproductive cost to the helper (B and C in offspring gained or lost, unitless counts). Altruism is favored when rB exceeds C.
On the MCAT: Passages describe alarm calls, sterile worker bees or ants, or helpers at the nest. You are asked which relative a helper favors (usually the closer one) or why a costly behavior persists. Altruism toward unrelated individuals is explained separately, by reciprocal altruism.
Common confusion: Inclusive fitness is not survival of the individual or the good of the species. Selection acts on the spread of alleles, and the helper need not be aware of it. Direct fitness is only one part of the total.
Sources
- Evolutionary Analysis
- Campbell Biology
- Psychology (Myers)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.