Genetical considerations also play an important role from a totally different angle of behavioral biology. The ultimate explanation for any behavioral trait has to refer to its relevance for fitness, which means that reproductive success is the ultimate 'coinage' to measure an organism's success in life. Fitness, as understood by evolutionary biologists and behavioral ecologists, refers to the proportion of one's own genetic material, relative to the contribution of one's competitors, in the genome of the next generation. In order to improve one's fitness, an organism, specifically an animal, has two possibilities: the direct way, also called improvement of direct or individual fitness, by producing as many successful offspring as possible, caring for them, etc.; or the indirect way, which goes via one's kin. To understand the improvement of indirect fitness, we have to bear in mind that the coefficient of relatedness is also an expression of shared genetic material. We are all related to our full offspring by 50% (half from the self's genome, half from the spouse). Full siblings are also related to each other by 50% because on average they will get half of their father's, half of their mother's genome, and the probability for each of them to get the same, or the complementary, chromosome of each parent again is 50%. Should our full siblings have children, then we would be related to each of our nephews and nieces by 25%. This is the basis for the so-called kin selection process, which means that selection not only favors those of us who contribute a lot to their direct fitness, but also those that help their kin to raise offspring successfully. The closer one's relatedness to the recipient of our good deed, the more we profit ourselves by means of improving our indirect fitness value. Thus, we have to consider both individual and kin selection when discussing positive or negative effects of any behavioral traits in evolutionary terms. This phenomenon is at the basis of the development ofthe social system in many species, and also has to be considered in population genetic recommendations and analyses (see below).
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