Quantum evolution
Encyclopedia
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).
George Gaylord Simpson
George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...
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
Sewall Wright
Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of...
theory of random genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
. 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
Deme (biology)
In biology, a deme is a term for a local population of organisms of one species that actively interbreed with one another and share a distinct gene pool...
from one stable adaptive peak to another on the adaptive fitness landscape
Fitness landscape
In evolutionary biology, fitness landscapes or adaptive landscapes are used to visualize the relationship between genotypes and reproductive success. It is assumed that every genotype has a well-defined replication rate . This fitness is the "height" of the landscape...
. However in his Major Features of Evolution (1953) Simpson wrote that it was still controversial
- "whether prospective adaptation as prelude to quantum evolution arises adaptively or inadaptively. It was concluded above that it usually arises adaptively . . . . The precise role of, say, genetic drift in this process thus is largely speculative at present. It may have an essential part or none. It surely is not involved in all cases of quantum evolution, but there is a strong possibility that it is often involved. If or when it is involved, it is an initiating mechanism. Drift can only rarely, and only for lower categories, have completed the transition to a new adaptive zone." (p. 390)
This preference for adaptive over inadaptive forces lead Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
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).