later books, including Natural Selection: Domains, Levels and Challenges, Williams softened his views on group selection, recognizing that clade selection, trait group selection and multilevel selection did sometimes occur in nature, something he had earlier thought to be so unlikely it could be safely ignored.

Williams became convinced that the genic neo-Darwinism of his earlier years, while essentially correct as a theory of microevolutionary change, could not account for evolutionary phenomena over longer time scales, and was thus an "utterly inadequate account of the evolution of the Earth's biota" (1992, p. 31). In particular, he became a staunch advocate of clade selection - a generalisation of species selection to monophyletic clades of any rank - which could potentially explain phenomena such as adaptive radiations, long-term phylogenetic trends, and biases in rates of speciation/extinction. In Natural Selection (1992), Williams argued that these phenomena cannot be explained by selectively-driven allele substitutions within populations, the evolutionary mechanism he had originally championed over all others. This book thus represents a substantial departure from the position of Adaptation and Natural Selection.