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An altruistic behavior is one in which an individual (the donor) performs an action that helps another animal (the recipient) with no apparent advantage to itself. Natural selection should eliminate such behaviors, yet there are many examples (alarm calling in squirrels, helpers at the nest in scrub jays, sterile worker castes in honey bees etc.) in which animals appear to cooperate despite an apparent disadvantage to the donor. In the example at hand, a good strategy might be for the donor to take information from the other bee without providing any itself. Some of these behaviors can be explained by the fact that the recipient of the "altruistic" behavior is a relative of the donor. In order to understand why this should be the case, it is first important to define the coefficient of relatedness.
The coefficient of relatedness (r) between two individuals is defined as the percentage of genes that those two individuals share by common descent. That may seem a little complicated, but it is actually easier to calculate than to define. Consider the simplest example of calculating the coefficient of relatedness of a parent and its offspring in a diploid system. The offspring inherit 1/2 of their genome from a particular parent, so that they will have a coefficient of relatedness of 0.5. If you take this one generation further, the offspring's offspring (grandoffspring) will have 1/2 of the genome of its parent, and consequently 1/2*1/2=1/4 of its genome from its grandparent. Thus, a grandparent and grandoffspring have a coefficient of relatedness (r) of .25. In general r =.5n where n is the number of "generational links" (Campbell 1993). Some more examples are shown in the figure on the right. |
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So, how is this "related" to altruism? W.D. Hamilton (1964. The evolution of social behavior. Journal of Theoretical Biology 7:1-52) proposed the concept of inclusive fitness which incorporates the potential costs and benefits of performing a given act to an individual and any of its genetic relatives. Thus, from an inclusive fitness point of view an individual is worth the equivalent of two of its siblings, or eight of its cousins, etc.. Consequently, an action that may be detrimental to the donor would still be beneficial to that donor's fitness if the action greatly benefits some of its relatives.
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Kin selection is the evolutionary mechanism that selects for those behaviors that increase the inclusive fitness of the donor. A well known example of kin selection in operation is the study of alarm calls in squirrels by Paul Sherman (1977. Nepotism and the evolution of alarm calls. Science 197:1246-1253). In this study, Sherman studied the likelihood of males (who do not nest near genetic relatives) and females (who do nest near genetic relatives) to give alarm calls that warn others of predators while placing caller at a greater risk of attack. It turns out that males are less likely to give such calls than females, thus supporting the kin selection hypothesis. |
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