Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

When the British geneticist W.D. Hamilton studied the evolution of altruistic behaviour in the nineteen sixties, he found that it is useful to differentiate two types of fitness, inclusive fitness and direct fitness (Hamilton 1964b). Inclusive fitness is most commonly considered in connection with altruistic behaviour (see IV.8.3); however, its importance extends to other areas of evolutionary biology. Basically, it can be stated that 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

rb>c,

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 (see also IV.8.3) either by producing a greater number of descendants or by assisting its relatives (Russell & Hatchwell 2001; Oli 2003).