Let me quote more extensively from Koonin’s paper:
Evolution proceeds by fixation of the rare beneficial variations and elimination of deleterious variations: this is the process of natural selection that, along with random variation, is the principal driving force of evolution according to Darwin and the Modern Synthesis. Natural selection which is, obviously, akin to and inspired by the ‘invisible hand’ (of the market) that ruled economy according to Adam Smith, was the first mechanism of evolution ever proposed that was simple, plausible, and did not require any mysterious innate trends. As such, this was Darwin’s second key insight. The founders of population genetics, in particular, Sewall Wright, emphasized that chance could play a substantial role in the fixation of changes during evolution not only in their emergence, via the phenomenon of genetic drift that entails random fixation of neutral or even deleterious changes. Population-genetic theory indicates that drift is particularly important in small populations that go through bottlenecks (6,16). However, the Modern Synthesis, in its ‘hardened’ form (13), effectively, rejected drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution (17).
The beneficial changes that are fixed by natural selection are ‘infinitesimally’ small, so that evolution proceeds via the gradual accumulation of these tiny modifications. Darwin insisted on strict gradualism as an essential staple of his theory: ‘Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being . . . If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.’ [(1), chapter 6]. Even some contemporaries of Darwin believed that was an unnecessary stricture on the theory. In particular, the early objections of Thomas Huxley are well known: even before the publication of the Origin Huxley wrote to Darwin ‘‘You have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly’ (18).
Now, let’s ponder two important questions.
Why did Darwin insist on strict gradualism as an essential staple of his theory?
Why did most proponents of the Modern synthesis reject drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution?