The idea, put forward independently by Motoo Kimura in 1968 and King and Jukes in 1969, that the vast majority of evolutionary changes at the molecular level are selectively neutral. Kimura argued that the rate of evolution of a set of mammalian proteins was occurring much too fast to be caused by natural selection given the cost of selection. The strict neutral theory, which Kimura championed for the rest of his career, has generally fallen out of favor to explain molecular evolution. (Natural selection, nearly neutral theory, and molecular drive are currently invoked to explain changes at the molecular level.) None the less, as Marty Kreitman, an influential selectionist has pointed out, the neutral theory provides a good null model for studies of molecular evolution.

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