Fitness interdependence is a concept in evolutionary biology and behavioral ecology that describes the degree to which the reproductive success of one organism depends on that of another. Other terms that have been used for it^[1] are pseudo-reciprocity, byproduct reciprocity, partnership, group augmentation, irreplaceability and vested interests. It has been proposed as a unifying framework for explaining cooperation and apparent altruism in cases that are not easily accounted for by kin selection or reciprocal altruism. The framework formalises the intuition that an individual can profit, in fitness terms, by investing in the welfare of a partner whose continued survival or reproduction confers passive benefits on the actor. Where such interdependence is strong, cooperation can be evolutionarily stable without requiring genetic relatedness, repeated interaction, or reputation-based reward.^[2][3]

The two dominant explanations for cooperation among animals are Hamilton's rule, under which altruism is favored when the benefit to a recipient devalued by genetic relatedness exceeds the cost to the actor (rb − c > 0), and reciprocal altruism, under which costs are recovered through return acts by the same recipient.^[2] Both have been productive but leave a residue of cases that neither explains. Roberts notes that in social insects, cooperatively breeding birds and cooperatively breeding mammals, "relatedness does not explain variance in helping behaviour".^[2] Reciprocity has likewise drawn what Roberts calls "increasing dissatisfaction", with empirical work failing to recover the contingent tit-for-tat dynamics that the theory predicts.^[2] In humans, both close friendships and food-sharing in hunter-gatherer societies have proven hard to fit to a contingent-exchange model.^[2][4]

These anomalies prompted a search for what Roberts termed "the holy grail of cooperation research": a route to cooperation that does not require active reciprocation.^[2] Earlier candidates included pseudoreciprocity,^[5] by-product mutualism,^[6][7] and group augmentation.^[8] The interdependence framework is in part a synthesis of these earlier proposals.^[2]

Roberts defines an actor A’s stake in a recipient B as the proportionality between changes in their fitnesses:^[2]

${\displaystyle s={\frac {w'_{A}-w_{A}}{w'_{B}-w_{B}}}={\frac {\Delta w_{A}}{\Delta w_{B}}}}$

where w denotes fitness before an interaction and w′ denotes fitness after. Where an altruistic act costs the actor c and benefits the recipient b, altruism is favored when:

${\displaystyle sb-c>0}$

This generalises Hamilton's rule: substituting genetic relatedness r for s recovers the inequality rb − c > 0.^[2] Roberts presents kinship and reciprocity as special cases of the stakeholder model — kinship being a stake at the genetic level, reciprocity being a stake mediated by the probability of return investment.^[2]

Category:Evolutionary biology Category:Behavioral ecology Category:Altruism Category:Evolutionary game theory