Altruism consists of unselfish/selfless acts in which the altruist puts other individual’s interests before their own. The general expectation is that the evolution of altruistic traits is not compatible with selfish gene theory. How could such a mechanism reinforce a behavior jeopardizing ones own interests at the expense of another's?
Units of Selection. Group selection. Altruistic behavior could be rooted through group selectionist thinking where altruistic traits favor reproductive success of a group at the expense of that of an individual.
Self-sacrificing Behavior: e.g. vampire bats, guard bees, Cooperative breeding, e.g. African Kingfishers, Acorn Woodpeckers, Florida Scrub Jay;
An individual behaves in such a way as to enhance the reproduction of another individual, at a cost to its own fitness. (e.g., Sterile workers in social insects who give up all reproduction for the benefit of their mother queen. Mutual or delayed benefits can’t account for this one: sterile workers never get to produce any daughters.)
Simple Egoistic Solutions for Individuals: Current or delayed benefits. Young may remain with their parents and help them or group members raise more offspring, rather than breeding themselves (e.g., ostriches, some primates). Helpers may gain useful experience in raising their own offspring or have an incrased chance to inherit a valuable breeding territory.
Complex Egoistic Solutions for Individuals: Prisoner's Dilemma: e.g. Pukekos (Porphyrio porphyrio)
Inclusive Fitness: An individual fitness solution to the paradox goes as follows: genes are favored that produce a disproportionate propagation of ones genes to subsequent generations even if it is at the expense of the individual. This may occur directly through either reproducing personally or indirectly by encouraging the reproduction of close relatives who share many of ones genes. Under those circumstances altruistic behaviors should primarily be directed towards close relatives who share many of the genes as compared to non-relatives who share fewer of them. Inclusive fitness refers to the sum of an individual's Direct Fitness (probability of reproductive success of one's own offspring) and its Indirect Fitness (probability of reproductive success of non-descendant relatives). Altruistic acts are frequently directed towards relatives. <Kin Selection>: selection for traits that lower an individual's personal fitness, but raise a relative's fitness. Recipient (related) kin share genes with the altruistic individual and are thus genetic extensions of them. Kin selection and inclusive fitness can explain sociality through increased survivorship of relatives, increased inclusive fitness, altruism, and delayed maturation or breeding.