Hamilton (1964) outlined two ways in which kin selection
altruism could be favoured.
Firstly, if individuals have the capacity to recognize kin (kin
recognition) and to adjust their behaviour on the basis of kinship
(kin discrimination), then the average relatedness of the
recipients of altruism could be high enough for this to be
favoured. Because of the facultative nature of this mechanism, it
is generally regarded that kin recognition and discrimination are
unimportant except among 'higher' forms of life (although there is
some evidence for this mechanism among protozoa). A special case of
the kin recognition/discrimination mechanism is the hypothetical
'green beard', where a gene for social behaviour also causes a
distinctive phenotype that can be recognised by other carriers of
the gene. Hamilton's discussion of greenbeard altruism serves as an
illustration that relatedness is a matter of genetic similarity and
that this similarity is not necessarily caused by genealogical
closeness (kinship).
Secondly, even indiscriminate altruism may be favoured in
so-called viscous populations, i.e. those characterized by low
rates or short ranges of dispersal. Here, social partners are
typically genealogically-close kin, and so altruism may be able to
flourish even in the absence of kin recognition and kin
discrimination faculties. This suggests a rather general
explanation for altruism. Directional selection will always favor
those with higher rates of fecundity within a certain population.
Social individuals can often ensure the survival their own kin by
participating in, and following the rules of a group.
from the web page
encyclopedia.thefreedictionary.com/Kin+selection
