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
Alternatives to evolution by natural selection through a population when they were beneficial, helped by natural selection. This represented the germ of the modern synthesis, and the beginning of the end for mutationism as an evolutionary force. Contemporary biologists accept that mutation and selection both play roles in evolution; the mainstream view is that while mutation supplies material for selection in the form of variation, all non-random outcomes are caused by natural selection. Masatoshi Nei argues instead that the production of more efficient genotypes by mutation is fundamental for evolution, and that evolution is often mutation-limited. The endosymbiotic theory implies rare but major events of saltational