The Evolution of Cooperation 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
The Selfish Gene gene is the object of selection, usefully completes and extends the explanation of evolution given by Charles Darwin before the basic mechanisms of genetics were understood. According to the ethologist Alan Grafen, acceptance of adaptionist theories is hampered by a lack of a mathematical unifying theory and a belief that anything in words alone must be suspect. According to Grafen, these difficulties along with an initial conflict with population genetics models at the time of its introduction "explains why within biology the considerable scientific contributions it ["The Selfish Gene"] makes are seriously underestimated, and why it is viewed mainly as