"An animal's behaviour tends to maximize the survival of the genes 'for' that behaviour, whether or not those genes happen to be in the body of the particular animal performing it." - from WikipediaSee also: Dawins, "The Extended Phenotype: The Long Reach of the Gene"
KIN SELECTION: The individual's only purpose (in the bigger biological picture) is to ensure that his genes are successfully passed on -- wherever they may be located.
[S]ome organisms tend to exhibit strategies that favor the reproductive success of their relatives, even at a cost to their own survival and/or reproduction.... [A] gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behavior, 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. "- from WikipediaSee also: Kin recognition
GAME THEORY: Interactions between individuals and between populations can be accounted for to a large degree by concepts from game theory. Game thoery:
...studies strategic interactions between agents. In strategic games, agents choose strategies which will maximize their return, given the strategies the other agents choose. The essential feature is that it provides a formal modelling approach to social situations in which decision makers interact with other agents. - from Wikipedia"In biology, the aim of the game is to successfully pass on one's genes to the next generation. Oftentimes, there are losers as well as winners.
See also: Maynard Smith, "Evolution ahd the Theory of Games"
COEFFICIENT OF RELATIONSHIP: Individuals in different populations share differing amounts of genes with their relations depending on the mating systems within those populations. The individuals of a population in which mating between close relatives (i.e. inbreeding) occurs share more genes (i.e. are MORE related to one another) than individuals of a population in which outbreeding occurs more frequently.
"In population genetics, Sewall Wright's coefficient of relationship or relatedness is the probability that at a random locus, the alleles there will be identical by descent." - from WikipediaSee also: Demystifying Inbreeding Coefficients