An enduring controversy in evolutionary biology is the genetic basis of adaptation. Darwin emphasized "many slight differences" as the ultimate source of variation to be acted upon by natural selection. In the early 1900's, this view was opposed by "Mendelian geneticists," who emphasized the importance of "macromutations" in evolution. The Modern Synthesis resolved this controversy, concluding that mutations in genes of very small effect were responsible for adaptive evolution.

A decade ago, Allen Orr and Jerry Coyne reexamined the evidence for this neo-Darwinian view and...