I wonder why you were initially downvoted? Anyway, to quote from Williams: "I see no reason why a conscious motive need be involved. It is necessary that help provided to others be occasionally reciprocated if it is to be favored by natural selection. It is not necessary that either the giver or the receiver be aware of this. ... Simply stated, an individual who maximizes his friendships and minimizes his antagonisms will have an evolutionary advantage, and selection should favor those characters that promote the optimization of personal relationships."

Robert Trivers expanded this idea and you can read his paper, "The Evolution of Reciprocal Altruism", here: http://www.cdnresearch.net/pubs/others/trivers_1971_recip.pd... Since then the evidence has continued to grow in support of the idea (at least in primates--birds and fish that Trivers suggest don't have as much evidence).

http://lesswrong.com/lw/kw/the_tragedy_of_group_selectionism... is a nice post on why group selection theories tend to be wrong by default and why you shouldn't really pay attention to them when they come up.

For people interested in evolutionary psychology I recommend Robert Wright's The Moral Animal, which does a good job covering the history of the field and offering explanations for a variety of behaviors. http://www.amazon.com/Moral-Animal-Science-Evolutionary-Psyc...