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
Gene-centered view of evolution views selection as working on many levels, and has called attention to a hierarchical perspective of selection. Gould also called the claims of "Selfish Gene" "strict adaptationism", "ultra-Darwinism", and "Darwinian fundamentalism", describing them as excessively "reductionist". He saw the theory as leading to a simplistic "algorithmic" theory of evolution, or even to the re-introduction of a teleological principle. Mayr went so far as to say "Dawkins' basic theory of the gene being the object of evolution is totally non-Darwinian." Gould also addressed the issue of selfish genes in his essay "Caring groups and selfish genes". Gould acknowledged that Dawkins was