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 to make some critical distinctions, in particular, the difference between group selection for group advantage and group selection conveying individual advantage. A good deal of objection to "The Selfish Gene" stemmed from its failure to be always clear about "selection" and "replication". Dawkins says the gene is the fundamental unit of selection, and then points out that selection doesn't act directly upon the gene, but upon 'vehicles' or 'extended phenotypes'. Stephen Jay Gould took exception to calling the gene a 'unit of selection' because selection acted only upon phenotypes. Summarizing the Dawkins-Gould difference of view, Sterelny says: The word 'cause'