It is the great irony of modern evolutionary genetics that the spirit of explanation has moved more and more towards optimal adaptation, while the technical developments of population genetics of the past 30 years have been increasingly to show the efficacy of non adaptive forces in evolution.
"A natural selection" Nature May 11,1989 p.107
The ‘neutral theory’ proposed that most evolutionary changes at the molecular level were caused by random genetic drift rather than by natural selection. Note that the neutral theory classifies new mutations as deleterious, neutral, and advantageous. Under this classification, the rate of mutant substitutions in evolution can be formulated by the stochastic theory of population genetics. Kimura's theory was simple and elegant, yet I was not quite satisfied with it, because I thought that natural selection was not as simple as the mutant classification the neutral theory indicated, and that there would be border-line mutations with very small effects between the classes. I thus went ahead and proposed the nearly neutral theory of molecular evolution in 1973. The theory was not simple, and much more complicated, but to me, more realistic, and I have been working on this problem ever since.
Current Biology, August 21, 2012
HatTip: Jerry Coyne (student of Lewontin): Dick Lewontin and Tomoko Ohta nab the Crafoord Prize