Inclusive fitness is one of two components of fitness, direct fitness and inclusive fitness.. Direct fitness takes into consideration reproductive success only of the particular individual, while inclusive fitness considers the reproductive success of both the particular individual and of his relatives.
For example, if the population contains an individual who helps his siblings, these individuals have, on an average, greater fitness than the other members of the particular population, even if they leave behind the same number of descendants. Their nephews and nieces have, in any case, at least ¼ of their genes in common because of their relatedness (the fraction of common genes can often be greater as a consequence of relatedness, as the relatives of particular individuals could have reproduced together in previous generations). From the standpoint of evolution it thus makes no difference if an individual assists in reproducing himself or two of his direct relatives or siblings (with whom he has at least ½ of their genes in common because of their relatedness) or 4 descendants of his descendant or sibling. Altruistic behaviour of individuals is thus worthwhile if
where r is the relationship of mutually assisting individuals, basically indicating how much larger will be the probability that an allele of the gene for altruistic behaviour will be shared by two specific individuals than that it will be shared by two randomly selected individuals in the population, b – the advantage that altruistic behaviour provides to the assisted individual and c – the cost that the assisting individual must pay for assisting. The price and the advantage are measured in terms of the relevant biological currency, i.e. in the biological fitness. Thus, under certain conditions, an organism can increase its fitness either by producing a greater number of descendants or by assisting its relatives.