Kin Selection, the first proposed explanation for the existance of altruism,
explains how altruism may arise between close relatives.
When adult animals risk their life to distract potential predators
away from their offspring they are exhibiting this form of altruism.
Such behaviour is evolutionarily stable since although it decreases
an animal's chances of survival, it increases the chances for its offspring.
Although most obvious in the case of direct offspring, kin selection can
operate between any animals that are relatives in proportion to their
coefficient of relatedness (the proportion of genes that two
individuals share by common descent). Any family relationship can be expressed
in these terms - so a cousin is as close as a grandparent but half as close as a
sibling, etc. Where the exact relationship is unknown to the animals involved,
is seems likely that they attempt to estimate the degree of relatedness on the
basis of apparent similarity.
Rothstein has argued that in most species kin selection is rarely completely separable from
reciprocal altruism because of the tendency of relatives to
live together and interact with one another.
William Hamilton: (1964) The Genetical Evolution of Social Behaviour, Journal of Theoretical Biology 7 pp.1-52
Steven Rothstein: (1980) Reciprocal altruism and kin selection are not clearly separable phenomena, Journal of Theoretical Biology 87 pp.255-261