Before we proceed to discuss kin selection we should define the term altruism. It refers to the behaviour pattern of an individual in the population for the benefit of other members. Probably genes for altruism regulate the behaviour of such individuals. The local warning signal given by a bird to alert the other members of the flock or a honey bee stinging an intruder in order to defend its hive, are a couple of examples of altruism. In both the instances the altruistic individual may not survive: the signaller may invite the attention of the predator and the  stinging bee usually dies. It should be obvious to you that the death of these individuals would result in the elimination of concerned genes for such behaviour. Critics of Darwin argue that although altruism as a behaviour pattern is an adaptation, the fact that altruists are victims of such behaviour refutes the concept-that natural selection would promote favourable genes in the population.

A British biologist, W.D. Hamilton, effectively theorised that altruism is not an evidence against natural selection and proposed the term kin selection to explain altruistic behaviour. The term kin selection could be equated to natural selection when we are considering the kin or relatives of an individual. For instance a mother expending energy suckling or caring for her children, only ensures the reproductive success of her own genes through her progeny. Kin selection essentially favours such altruistic behaviour when the risk taken or energy spent by an individual is more than compensated by the benefits accrued by the relatives.