and even altruism, within the Darwinian theory is a central issue in the theory of cooperation. Modern developments Darwin's explanation of how evolution works is quite simple, but the implications of how it might explain complex phenomena are not at all obvious; it has taken over a century to elaborate (see modern synthesis). Explaining how altruism – which by definition reduces personal fitness – can arise by natural selection is a particular problem and the central theoretical problem of sociobiology. A possible explanation of altruism is provided by the theory of group selection (first suggested by Darwin himself while grappling with the issue of social insects), which argues that natural selection can act on groups: groups that are more successful – for any reason, including learned behaviors – will benefit the individuals of the group, even if they are not related. It has had a powerful appeal but has not been fully persuasive, in part because of difficulties regarding cheaters that participate in the group without contributing. Another explanation is provided by the genetic kinship theory of William D. Hamilton: if a gene causes an individual to help other individuals that carry copies of that gene, then the gene has a net benefit even with the sacrifice of a few individuals. The classic example is the social insects, where the workers – which are sterile, and therefore incapable of passing on their genes – benefit the queen, who is essentially passing on copies of "their" genes. This is further elaborated in the "selfish gene" theory of Richard Dawkins, that the unit of evolution is not the individual organism, but the gene. (As stated by Wilson: "the organism is only DNA's way of making more DNA.") However, kinship selection works only where the individuals involved are closely related; it fails to explain the presence of altruism and cooperation between unrelated individuals, particularly across species. In a 1971 paper, Robert Trivers demonstrated how reciprocal altruism can evolve between unrelated individuals, even between individuals of entirely different species. And the relationship of the individuals involved is exactly analogous to the situation in a certain form of the Prisoner's Dilemma. The key is that in the iterated Prisoner's Dilemma, or IPD, both parties can benefit from the exchange of many seemingly altruistic acts. As Trivers says, it "take[s] the altruism out of altruism." The Randian premise that self-interest is paramount is largely unchallenged but