Also, passing on genes to offspring can aid reproduction of other individuals who are likely to carry the same gene. Hamilton's argument was that species carrying a gene for altruism could evolve under Darwinian selection if the altruist's nature granted a genetic relative that shared the same gene to bear offspring more than it would otherwise have done. He is implying that an individual should always naturally prefer to aid close kin rather than distant relatives. .
The "Kin Selection Theory" can be outlined by using "Hamilton's Rule"; a gene for altruism will develop when rB >C where B is the benefit gained by the receiver, C is the cost suffered by the altruist, and r is the genetic relatedness of the altruist to the beneficiary. Altruism can evolve in a population if a helper can make up for losing C offspring by adding to the population B offspring bearing a fraction r of its genes. For example, a lioness with a well-nourished cub gains inclusive fitness by nursing a deprived cub of a close kin, let's say a full sister, because the benefit to her sister more that atones the loss to herself, since the survival contingency of her own, well-fed cub is slightly reduced. Hamilton's Rule will be computed as follows: (.5 x 1) > .25, given that the genetic relatedness between the two sisters is .5. The bottom line is that the genes for altruism expand by giving aid to copies of themselves. .
This theory can only explain altruism when the donor and recipient are akin to each other. .
But what about those situations where they aren't related? Packer (1977) struggled to understand the evolution of cooperation in different taxa, and studying primates was one of the most complicated tasks. He then discovered reciprocal altruism among male olive baboons (Papio Anhuis). Adult male P. Anhui's form short-term coalitions with each other and is initiated by an enlisting partner and completed by fighting the opponent.