George C. Williams (biologist) should only be used where it is really necessary", and, that, when it is necessary, selection among genes or individuals would in general be the preferable explanation for it. He elaborated this view in later books and papers, which contributed to the development of a gene-centered view of evolution; Richard Dawkins built on Williams' ideas in this area in the book "The Selfish Gene". Williams was also well known for his work on the evolution of sex, and was an advocate of evolutionary medicine. In later books, including "Natural Selection: Domains, Levels and Challenges", Williams softened his views on group
The Selfish Gene and Natural Selection" (1966), which argued that altruism is not based upon group benefit "per se", but is a result of selection that occurs "at the level of the gene mediated by the phenotype" and any selection at the group level occurred only under rare circumstances. This approach was developed further during the 1960s by W. D. Hamilton and others who opposed group selection and selection aimed directly at benefit to the individual organism: Despite the principle of 'survival of the fittest' the ultimate criterion which determines whether [a gene] "G" will spread is not whether the behavior is to