Quantum evolution is a component of

George Gaylord Simpson George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern synthesis, contributing '' Tempo ...

's multi-tempoed theory of

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

proposed to explain the rapid emergence of higher taxonomic groups in the

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

. 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 ''bradytelic'' (slow tempo), ''horotelic'' (medium 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 In physics, a quantum (: quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a property can be "quantized" is referred to as "the hypothesis of quantization". This me ...

" therefore refers to an "all-or-none reaction", where transitional forms are particularly unstable, and thereby perish rapidly and completely. Although quantum evolution may happen at any taxonomic level, it plays a much larger role in "the origin taxonomic units of relatively high rank, such as

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

, orders, and classes."Simpson, G. G. (1944). '' Tempo and Mode in Evolution''. New York: Columbia Univ. Press
p. 206.
/ref>

Quantum evolution in plants

Usage of the phrase "quantum evolution" in plants was apparently first articulated by Verne Grant in 1963 (pp. 458-459). He cited an earlier 1958 paper by Harlan Lewis and Peter H. Raven, wherein Grant asserted that Lewis and Raven gave a "parallel" definition of quantum evolution as defined by Simpson. Lewis and Raven postulated that species in the Genus Clarkia had a mode of speciation that resulted

...as a consequence of a rapid reorganization of the chromosomes due to the presence, at some time, of a genotype conducive to extensive chromosome breakage. A similar mode of origin by rapid reorganization of the chromosomes is suggested for the derivation of other species of Clarkia. In all of these examples the derivative populations grow adjacent to the parental species, which they resemble closely in morphology, but from which they are reproductively isolated because of multiple structural differences in their chromosomes. The spatial relationship of each parental species and its derivative suggests that differentiation has been recent. The repeated occurrence of the same pattern of differentiation in Clarkia suggests that a rapid reorganization of chromosomes has been an important mode of evolution in the genus. This rapid reorganization of the chromosomes is comparable to the systemic mutations proposed by Goldschmidt as a mechanism of
macroevolution Macroevolution comprises the evolutionary processes and patterns which occur at and above the species level. In contrast, microevolution is evolution occurring within the population(s) of a single species. In other words, microevolution is the ...
. In Clarkia, we have not observed marked changes in physiology and pattern of development that could be described as macroevolution. Reorganization of the genomes may, however, set the stage for subsequent evolution along a very different course from that of the ancestral populations

Harlan Lewis refined this concept in a 1962 paper where he coined the term "Catastrophic Speciation" to describe this mode of speciation, since he theorized that the reductions in population size and consequent inbreeding that led to chromosomal rearrangements occurred in small populations that were subject to severe drought. Leslie D. Gottlieb in his 2003 summary of the subject in plants stated

we can define quantum speciation as the budding off of a new and very different daughter species from a semi-isolated peripheral population of the ancestral species in a cross-fertilizing organism...as compared with geographical speciation, which is a gradual and conservative process, quantum speciation is rapid and radical in its phenotypic or genotypic effects or both.

Gottlieb did not believe that sympatric speciation required

disruptive selection In evolutionary biology, disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait in ...

to form a reproductive isolating barrier, as defined by Grant, and in fact Gottlieb stated that requiring disruptive selection was "unnecessarily restrictive" in identifying cases of sympatric speciation. In this 2003 paper Gottlieb summarized instances of quantum evolution in the plant species Clarkia, Layia, and Stephanomeria.

Mechanisms

According to Simpson (1944), quantum evolution resulted from

Sewall Wright Sewall Green Wright ForMemRS HonFRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. He was a founder of population genetics alongside ...

's model of random genetic drift. Simpson believed that major evolutionary transitions would arise when small populations, that were isolated and limited from

gene flow In population genetics, gene flow (also known as migration and allele flow) is the transfer of genetic variation, genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent ...

, would fixate upon unusual gene combinations. This "inadaptive phase" (caused 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 this mechanism 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."

This preference for adaptive over inadaptive forces led

Stephen Jay Gould Stephen Jay Gould ( ; September 10, 1941 – May 20, 2002) was an American Paleontology, paleontologist, Evolutionary biology, evolutionary biologist, and History of science, historian of science. He was one of the most influential and widely re ...

to call attention to the "hardening of the Modern Synthesis", a trend in the 1950s where adaptationism took precedence over the pluralism of mechanisms common in the 1930s and 40s.Gould, S. J. (1983)
"The hardening of the Modern Synthesis"
In Marjorie Grene, ed., ''Dimensions of Darwinism''. Cambridge UK: Cambridge University Press, pp. 71-93. Simpson considered quantum evolution his crowning achievement, being "perhaps the most important outcome of yinvestigation, but also the most controversial and hypothetical."

References

Sources

* Eldredge, Niles (1995). ''Reinventing Darwin''. New York: John Wiley & Sons. pp. 20-26. * *Gould, S. J. (1994)
"Tempo and mode in the macroevolutionary reconstruction on Darwinism"
''PNAS USA'' 91(15)
6764-71.
*Gould S.J. (2002). '' The Structure of Evolutionary Theory'' Cambridge MA: Harvard Univ. Press
pp. 529-31.
*Mayr, Ernst (1976). ''Evolution and the Diversity of Life''. Cambridge MA: Belknap Press
p. 206.
*Mayr, Ernst (1982). ''The Growth of Biological Thought''. Cambridge MA: Belknap Press. pp. 555
609-10.

External links

- Biographical sketch.
Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson
{{Evolution Evolutionary biology Modern synthesis (20th century) Rate of evolution

Quantum evolution in plants

Mechanisms

See also

References

Sources

External links