Quantum evolution is a component of George Gaylord Simpson's multi-tempoed theory of evolutionary change, proposed to explain the rapid emergence of higher taxonomic groups. According to Simpson, evolutionary rates differ from group to group and even among closely related lineages. These different rates of evolutionary change were designated by Simpson as horotelic (medium tempo), bradytelic (slow tempo), and tachytelic (rapid tempo). Quantum evolution differed from these styles of change in that it involved a drastic shift in the adaptive zones of certain classes of animals. The word "quantum" therefore refers to an "all-or-none reaction," where transitional forms are particularly unstable, and perished rapidly and completely. Although quantum evolution may happen at any taxonomic level (1953, 389), it plays a much larger role in "the origin taxonomic units of relatively high rank, such as families, orders, and classes." (1944, 206)
According to Simpson (1944) quantum evolution relied heavily upon Sewall Wright's theory of random genetic drift. Simpson believed that major evolutionary transitions would arise when small populations—isolated and limited from gene flow—would fixate upon unusual gene combinations. This "inadaptive phase" (by genetic drift) would then (by natural selection) drive a deme population from one stable adaptive peak to another on the adaptive fitness landscape. However in his Major Features of Evolution (1953) Simpson wrote that it was still controversial
This preference for adaptive over inadaptive forces lead Stephen Jay Gould to call attention to the "hardening of the Modern Synthesis," a trend in the 1950s where adaptationism took precedent over the pluralism of mechanisms common in the 1930s and 40s.
Simpson considered quantum evolution his crowning achievement, being "perhaps the most important outcome of [my] investigation, but also the most controversial and hypothetical." (1944, p. 206).