Introduction to Kin Selection


Some organisms tend to exhibit strategies that favour the reproductive success of their relatives, even at a cost to their own survival and/or reproduction. The classic example is a eusocial (highly social) insect colony, with sterile females acting as workers to assist their mother in the production of additional offspring. Many evolutionary biologists explain this by the theory of kin selection. Natural selection should eliminate such behaviours; however, there are many cases, such as alarm calling in squirrels, helpers at the nest in scrub jays, and sterile worker castes in honey bees, in which these animals cooperate despite an obvious disadvantage to the donor.

This sacrifice of individual success for the aid of other individuals is known as altruism.

There are thought to be four possible ‘routes’ to altruism – why it might arise, these are:

  • Kin selection – Keeping altruism in the family, possibly shared in the genes. Altruism within a family helps it to proliferate well.
  • Reciprocal altruism – ‘One good turn, deserves another.’ Altruism expressed by an individual is at some point returned. E.g. social grooming in primates, the individual doing the grooming is eventually groomed back.
  • Selfish mutualism – ‘What’s in it for me?’ Altruism which is expressed only because an individual also gains from it. E.g. feeding in house sparrows, they will call for help to break up large pieces of food which they are unable to carry alone thus losing some of the resource but gained more than they would have alone.
  • Group selection – ‘For the good of the group.’ Groups within a population – not necessarily family – which benefit by co-operation.

Kin Selection

John Maynard Smith described Kin Selection in 1964 as “…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.”

It goes on the idea that because similar genes are more prevalent within a family (either by kind [species] or by descent [ancestral]), any altruistic genes expressed within the family are more likely to become more prevalent within the entire species.

Kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals. 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 gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behaviour (altruistic genes), may nonetheless increase in frequency, because relatives often carry the same gene; this is the fundamental principle behind the theory of kin selection. According to the theory, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour.

Hamilton’s Rule

Whether or not altruism is favoured within a family or species depends on whether or not Hamilton’s rule is met:

Hamilton’s Rule: rB-C>0 or rearranged rB>C

Altruism is favoured when rB>C

  • C – The cost of displaying altruism, any disadvantages to the individuals.
  • B – Benefit to the individual(s) who receive aid.
  • r – The coefficient of relatedness. The probability that 2 individuals contain a gene identical by descent at the same locus. It has a value of 0-1.

Possible r values:

Relationship Coefficient of Relatedness
Identical Twins 1.0
Parent to an offspring 0.5
Siblings 0.5
Half Siblings 0.25
Unrelated individuals 0.0

Hamilton’s rule therefore predicts that we expect closer related individuals to express greater amounts of altruism. For example:

Would a mother warn her child of a predator, thus exposing herself as a target? If doing so has an arbitrary value of 2, but the benefit of saving the child of 5 then using Hamilton’s Rule the following must be true:

r(0.5) x B(5) – C(2) > 0

0.5 x 5 – 2 = 0.5

Thus as the value is greater than zero, altruism in this situation is favoured, would the same be true between half siblings? (r=0.25)

r(0.25) x B(5) – C(2) > 0

0.25 x 5 – 2 = -0.75

Because the result of Hamilton’s rule is less than 0, altruism in the same situation but a half-sibling attempting to warn a half-sibling, is not favoured.

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