To appear in Heams et al. (eds.), Handbook of Evolutionary Biology (provis. Title), Springer.

Please do not quote without permission. This is not the final version

an an altruistic leads to helpful behaviour at the expense of the altruistic individual’s capacity for survival and reproduction. Therefore, natural selection should in principle retain only the traits that favour the adaptation and reproduction of their carriers at the expense of traits that are harmful to their carriers. So how has a tendency to behave altruistically been able to evolve? William Hamilton came up with an ingenious solution when he invented the theory of kin selection (1964). Instead of thinking in terms of advantages for the individual carriers of altruistic traits, it is worth concentrating attention on the genes responsible for these behaviours and on the way in which these genes spread in the whole population. We know that individuals who are kin have a large part of their genes in common (in general, parents and children have 50% of genes in common, as do brothers and sisters). If there are genes that lead to altruistic behaviours in favour of an individual’s kin, the occasional disadvantage for the agent could be, from the gene’s point of view, compensated by an increase in the capacity for survival and reproduction of its close kin (given that there is high probability that they are carrying copies of the same genes). The theory of kin selection is very powerful in explaining self-sacrificing behaviour towards close kin. There are other explanatory models that we can appeal to for the whole series of other cooperative behaviours that occur between more far related individuals. For example, there is the theory of reciprocity (often called reciprocal altruism), which says that it is worth doing another a favour if one will have a good chance of being able to benefit in the future from a returned favour (Axelrod 1984, Trivers 1971). There is also the theory of indirect reciprocity, according to which it can be advantageous for an individual to invest energy doing a favour for another or for the community in order to gain a good reputation (Zahavi 1977). These different theories are complementary; it is possible that the evolution of social behaviour is the joint result of several of these mechanisms (Lehmann & Keller 2006). The behaviour of social insects such as bees is certainly due to kin selection because the worker bees are sisters. As for the marmots’ behaviour, it could be a mixture of kin selection and reciprocity. A final type of evolutionary explanation of highly social behaviours is the theory of group selection (D. S. Wilson 1975). This kind of selection does not operate at the level of the genes (in the way kin selection does), nor at the level of individual advantage in the long term (in the way direct and indirect reciprocity do), but at the level of the group. The idea is that highly social behaviours that are unfavourable from the individual point of view can spread in the population, provided they allow their group to survive better in a hostile environment; if groups composed of a great number of altruists survive better than groups composed uniquely of egoists, in total, it is possible that more altruists will survive (even if, from an individual point of view, it is better to be an egoist in a group of altruists). However, we should note that this theory is still controversial in evolutionary circles; it seems that it could be included in an selection.23 enlarged version of kin All these explanations reflect the fact that, in the course of evolution, animal species have developed different strategies to adapt to their environment; one set of strategies that has proved effective for the survival of species that live in groups consists precisely in adopting highly social behaviours – altruism, mutual aid. A certain number of evolutionary

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