In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness (how many of its own offspring it produces and supports) and the number of equivalents of its own offspring it can add to the population by supporting others. Advocates of inclusive fitness theory say that an organism can improve its overall genetic success by cooperative social behavior.
From the gene's point of view, evolutionary success ultimately depends on leaving behind the maximum number of copies of itself in the population. Until 1964, it was generally believed that genes only achieved this by causing the individual to leave the maximum number of viable offspring. However, in 1964 W. D. Hamilton proved mathematically that, because close relatives of an organism share some identical genes, a gene can also increase its evolutionary success by promoting the reproduction and survival of these related or otherwise similar individuals.
Belding's ground squirrel provides an example. The ground squirrel gives an alarm call to warn its local group of the presence of a predator. By emitting the alarm, it gives its own location away, putting itself in more danger. In the process, however, the squirrel protects its relatives within the local group (along with the rest of the group). Therefore, if protecting the other squirrels in the immediate area will lead to the passing on of more of the squirrel’s own genes than the squirrel could leave by reproducing on its own, then natural selection will favor giving the alarm call, provided that a sufficient fraction of the shared genes include the gene(s) predisposing to the alarm call. Further study has shown that the self-reported likelihood of risking one's life to save other's life is directly a function of the degree of genetic relatedness to the helper (Burnstein et al., 1994)
Synalpheus regalis, a eusocial shrimp, also is an example of an organism that seeks to increase its inclusive fitness. The larger defenders protect the young juveniles in the colony from outsiders. By ensuring the young's survival, the genes will continue to be passed onto future generations.
Inclusive fitness is more generalized than strict kin selection, which requires that the shared genes are identical by descent. Inclusive fitness is not limited to cases where kin are involved.
Read more about Inclusive Fitness: Hamilton's Rule, Inclusive Fitness and Altruism, Green-Beard Effects, Inclusive Fitness and Parental Care
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