Kin selection is the evolutionary
strategy that favours the reproductive
success of an organism's relatives, even
at a cost to the organism's own survival
and reproduction. Kin altruism is
altruistic behaviour whose evolution is
driven by kin selection. Kin selection
is an instance of inclusive fitness,
which combines the number of offspring
produced with the number an individual
can produce by supporting others, such
as siblings.
Charles Darwin discussed the concept of
kin selection in his 1859 book, The
Origin of Species, where he reflected on
the puzzle of sterile social insects,
such as honey bees, which leave
reproduction to their sisters, arguing
that a selection benefit to related
organisms would allow the evolution of a
trait that confers the benefit but
destroys an individual at the same time.
R.A. Fisher in 1930 and J.B.S. Haldane
in 1932 set out the mathematics of kin
selection, with Haldane famously joking
that he would willingly die for two
brothers or eight cousins. In 1964, W.D.
Hamilton popularised the concept and the
major advance in the mathematical
treatment of the phenomenon by George R.
Price which has become known as
"Hamilton's rule". In the same year John
Maynard Smith used the actual term kin
selection for the first time.
According to Hamilton's rule, kin
selection causes genes to increase in
frequency when the genetic relatedness
of a recipient to an actor multiplied by
the benefit to the recipient is greater
than the reproductive cost to the actor.
The rule is difficult to test but a
study of red squirrels in 2010 found
that adoption of orphans by surrogate
mothers in the wild occurred only when
the conditions of Hamilton's rule were
met. Hamilton proposed two mechanisms
for kin selection: kin recognition,
where individuals are able to identify
their relatives, and viscous
populations, where dispersal is rare
enough for populations to be closely
related. The viscous population
mechanism makes kin selection and social
cooperation possible in the absence of
kin recognition. Nurture kinship, the
treatment of individuals as kin when
they live together, is sufficient for
kin selection, given reasonable
assumptions about dispersal rates. Kin
selection is not the same thing as group
selection, where natural selection acts
on the group as a whole.
In humans, altruism is more likely and
on a larger scale with kin than with
unrelated individuals; for example,
humans give presents according to how
closely related they are to the
recipient. In other species, vervet
monkeys use allomothering, where related
females such as older sisters or
grandmothers often care for young,
according to their relatedness. The
social shrimp Synalpheus regalis
protects juveniles within highly related
colonies.
Historical overview 
Charles Darwin was the first to discuss
the concept of kin selection. In The
Origin of Species, he wrote clearly
about the conundrum represented by
altruistic sterile social insects that
This difficulty, though appearing
insuperable, is lessened, or, as I
believe, disappears, when it is
remembered that selection may be applied
to the family, as well as to the
individual, and may thus gain the
desired end. Breeders of cattle wish the
flesh and fat to be well marbled
together. An animal thus characterised
has been slaughtered, but the breeder
has gone with confidence to the same
stock and has succeeded.
In this passage "the family" and "stock"
stand for a kin group. These passages
and others by Darwin about "kin
selection" are highlighted in D.J.
Futuyma's textbook of reference
Evolutionary Biology and in E. O.
Wilson's Sociobiology.
The earliest mathematically formal
treatments of kin selection were by R.A.
Fisher in 1930 and J.B.S. Haldane in
1932 and 1955. J.B.S. Haldane fully
grasped the basic quantities and
considerations in kin selection,
famously writing "I would lay down my
life for two brothers or eight cousins".
Haldane's remark alluded to the fact
that if an individual loses its life to
save two siblings, four nephews, or
eight cousins, it is a "fair deal" in
evolutionary terms, as siblings are on
average 50% identical by descent,
nephews 25%, and cousins 12.5%. But
Haldane also joked that he would truly
die only to save more than a single
identical twin of his or more than two
full siblings. In 1955 he clarified:
Let us suppose that you carry a rare
gene that affects your behaviour so that
you jump into a flooded river and save a
child, but you have one chance in ten of
being drowned, while I do not possess
the gene, and stand on the bank and
watch the child drown. If the child's
your own child or your brother or
sister, there is an even chance that
this child will also have this gene, so
five genes will be saved in children for
one lost in an adult. If you save a
grandchild or a nephew, the advantage is
only two and a half to one. If you only
save a first cousin, the effect is very
slight. If you try to save your first
cousin once removed the population is
more likely to lose this valuable gene
than to gain it. … It is clear that
genes making for conduct of this kind
would only have a chance of spreading in
rather small populations when most of
the children were fairly near relatives
of the man who risked his life.
W. D. Hamilton, in 1963 and especially
in 1964, popularised the concept and the
more thorough mathematical treatment
given to it by George Price.
John Maynard Smith may have coined the
actual term "kin selection" in 1964:
These processes I will call kin
selection and group selection
respectively. Kin selection has been
discussed by Haldane and by Hamilton. …
By kin selection I mean the evolution of
characteristics which favour the
survival of close relatives of the
affected individual, by processes which
do not require any discontinuities in
the population breeding structure.
Kin selection causes changes in gene
frequency across generations, driven by
interactions between related
individuals. This dynamic forms the
conceptual basis of the theory of social
evolution. Some cases of evolution by
natural selection can only be understood
by considering how biological relatives
influence each other's fitness. Under
natural selection, a gene encoding a
trait that enhances the fitness of each
individual carrying it should increase
in frequency within the population; and
conversely, a gene that lowers the
individual fitness of its carriers
should be eliminated. However, a
hypothetical gene that prompts behaviour
which enhances the fitness of relatives
but lowers that of the individual
displaying the behaviour, may
nonetheless increase in frequency,
because relatives often carry the same
gene. According to this principle, the
enhanced fitness of relatives can at
times more than compensate for the
fitness loss incurred by the individuals
displaying the behaviour, making kin
selection possible. This is a special
case of a more general model, "inclusive
fitness". This analysis has been
challenged, Wilson writing that "the
foundations of the general theory of
inclusive fitness based on the theory of
kin selection have crumbled" and that he
now relies instead on the theory of
eusociality and "gene-culture
co-evolution" for the underlying
mechanics of sociobiology.
"Kin selection" should not be confused
with "group selection" according to
which a genetic trait can become
prevalent within a group because it
benefits the group as a whole,
regardless of any benefit to individual
organisms. All known forms of group
selection conform to the principle that
an individual behaviour can be
evolutionarily successful only if the
genes responsible for this behaviour
conform to Hamilton's Rule, and hence,
on balance and in the aggregate, benefit
from the behaviour.
Hamilton's rule 
Formally, genes should increase in
frequency when
where
r = the genetic relatedness of the
recipient to the actor, often defined as
the probability that a gene picked
randomly from each at the same locus is
identical by descent.
B = the additional reproductive benefit
gained by the recipient of the
altruistic act,
C = the reproductive cost to the
individual performing the act.
This inequality is known as Hamilton's
rule after W. D. Hamilton who in 1964
published the first formal quantitative
treatment of kin selection.
The relatedness parameter in Hamilton's
rule was introduced in 1922 by Sewall
Wright as a coefficient of relationship
that gives the probability that at a
random locus, the alleles there will be
identical by descent. Subsequent
authors, including Hamilton, sometimes
reformulate this with a regression,
which, unlike probabilities, can be
negative. A regression analysis
producing statistically significant
negative relationships indicates that
two individuals are less genetically
alike than two random ones. This has
been invoked to explain the evolution of
spiteful behaviour consisting of acts
that result in harm, or loss of fitness,
to both the actor and the recipient.
There has been little empirical support
for Hamilton's rule in wild animals, as
it is hard to quantify the costs and
benefits of different behaviours. The
first study to test Hamilton's rule
successfully was in 2010, involving a
wild population of red squirrels in
Yukon, Canada. The researchers found
that surrogate mothers would adopt
related orphaned squirrel pups but not
unrelated orphans. The researchers
calculated the cost of adoption by
measuring a decrease in the survival
probability of the entire litter after
increasing the litter by one pup, while
benefit was measured as the increased
chance of survival of the orphan. The
degree of relatedness of the orphan and
surrogate mother for adoption to occur
depended on the number of pups the
surrogate mother already had in her
nest, as this affected the cost of
adoption. The study showed that females
always adopted orphans when rB > C, but
never adopted when rB 
