Neutral theory does not deny the occurrence of natural selection. Hughes writes: "Evolutionary biologists typically distinguish two main types of natural selection:
purifying selection, which acts to eliminate deleterious mutations; and
positive (Darwinian) selection, which favors advantageous mutations. Positive selection can, in turn, be further subdivided into
directional selection, which tends toward fixation of an advantageous allele, and
balancing selection, which maintains a
polymorphism. The neutral theory of molecular evolution predicts that purifying selection is ubiquitous, but that both forms of positive selection are rare, whereas not denying the importance of positive selection in the origin of adaptations."
[9] In another essay, Hughes writes: "Purifying selection is the norm in the evolution of protein coding genes. Positive selection is a relative rarity — but of great interest, precisely because it represents a departure from the norm."
[10] A more general and more recent view of molecular evolution is presented by
Nei.
[8]