Altruism provides an evolutionary question. Individuals seek to maximize their fitness, and increase their genotypic representation in the population. This is done directly, by maximizing the number of offspring, and competition for resources. Altruistic behavior comes at a cost to fitness. Why be altruistic? To understand, one must realize that an individual maximizes fitness when he/she spreads the maximum number of their alleles to the next generation. Offspring represent half of the alleles of their parents, but what about nieces and nephews? They too, have a representation of the alleles of their uncles and aunts. For any two individuals related by descent there is a coefficient of relatedness. Parent and child share half their genes. Uncle and nephew share a quarter, and so on. For any individual, one may then infer that, in addition to direct fitness there is a value of indirect fitness- the fitness of their relatives by a factor of their relatedness to them. Individuals may behave altruistically and still benefit their own fitness if it benefits their indirect fitness. An animal will be expected to behave altruistically when the fitness benefit to the target individual/group by their relatedness is greater than the fitness costs.
Indirect Genetic Effects (IGES) arise when an individual's phenotype affects the fitness of one or more phenotypes in their social partners. IGEs have numerous social and evolutionary outcomes Altruism as defined above can be seen as one type of outcome of an IGE. IGEs can also produce feedback effects (when phenotypes affect their own fitness in others) and strongly affect the evolutionary rate of phenotypes. Indirect Genetic Effects can also drive the evolution of altruism in individuals who are not related by descent, since it acts on phenotypes (not genotypes).
Selection arising from altruism and IGEs are considered a type of social selection, which is different from natural selection, that being selection arising from the environment. In fitness models, the effects of natural and social selection on individuals must be considered separately.
Altruism is observed in humans perhaps more than any other species, besides eusocial animals. Humanity's closest relatives and ancestors lived in highly social environments, but for most of civilization we have lived in environments where the selection pressures on individuals far outweighs environmental selection pressures. In a situation with low environmental selection and strong social selection an altruistic phenotype that produces positive feedback would experience rapid evolution. Perhaps Indirect Genetic Effects can help in explaining selfless behavior in mankind.
