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Proposed by William D. Hamilton in 1964, and rightly considered one of the biggest breakthroughs in 20th century evolutionary biology, inclusive fitness refers to the sum of an individual's reproductive success, or "classical fitness", plus the effects the individual has on the reproductive success of his/her genetic relations.
Hamilton's equation is
rb-c > 0 for a gene for altruistic behaviour to spread in a population.
c = reproductive cost to the altruist
b = benefit to the recipient of the altruistic behavior
r = defined by Grafen in 1985 as the probability, above the population average, of the individuals sharing an altruistic gene. Commonly viewed as 'degree of relatedness'.