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How does natural selection favor individuals cooperating to produce a greater, more successful whole, even if it means never reproducing? William D. Hamilton’s theory of kin selection helps to answer this question. Kin selection is rooted in genes producing copies in two manners: direct fitness by rearing offspring or indirect fitness by helping to care for relatives who also share these genes ( [link] ). Therefore helping your relatives will always be advantageous. Natural selection favors this altruistic behavior when the cost of helping kin is less than the reproductive benefit achieved for the beneficiary of the altruism. This reproductive benefit can be evaluated by examining not only the benefit to the other individual, but also the degree of relatedness between the altruistic individual and the beneficiary ( [link] ) (Hamilton 1964). Thus kin selection explains how eusocial behavior could arise in sponge-dwelling shrimp. The benefit in this ratio is very large, since juveniles (the sole means to pass your copy of genes) are incapable of fighting for themselves due to the lack of claws. The non-reproductive defenders ensure that these juveniles would survive, exalting a cost for not reproducing that is greatly overshadowed with the ascent of the juveniles into adulthood. Since juveniles never leave their natal nest, there is a high coefficient of relatedness in the sponges. By living with relatives, members of the colony can alter their behavior within the colony as a juvenile, breeding female, or a large male (Agrawal 2001). For example, even though some juveniles are not offspring of the large male sponge-dwelling shrimp, they share some of the same genes, resulting in an indirect fitness benefit for the large males when they ensure the survival of the juveniles. Allozyme data collected by Duffy et al. prove that the majority of colony members are full siblings—allowing for kin selection and indirect fitness to take place due to a high degree of relatedness.
Hamilton’s rule is a mathematical formula in determining if altruistic indirect fitness can be selected for by kin selection. The formula is C-B(R)<0, with C as the cost of the action for the actor, B as the benefits the recipient obtains from the action, and R is the relatedness between the recipient and the actor. For instance, suppose that a diploid female forgoes the opportunity to reproduce, instead remaining on her parents’ nest to assist in the development in her siblings. The juveniles of this species require tremendous parental care in terms of feeding, thus would greatly benefit from the assistance of a sibling. This species is monogamous resulting in siblings having the same parents, a genetic relatedness (R) of ½. The cost of not reproducing would be forgoing the chance to produce two offspring. However the benefits the parents receive through the helper daughter would be increasing the clutch size to five offspring. Therefore (2)-(1/2)(5)= - ½ which is less than 0. Since the value is less than zero, the presence of a helper daughter, that forgoes the opportunity to mate, will be selected for under kin selection.
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