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