Neo-Darwinism
Neo-Darwinism, also called the modern evolutionary synthesis, generally denotes the integration of Charles Darwin's theory of evolution by natural selection, Gregor Mendel's theory of genetics as the basis for biological inheritance, and mathematical population genetics. Although this was not the historical meaning of the term neo-Darwinism, it has been the popular and scientific use of the expression since the synthesis of the 1930s. (See History.) Other terminology used synonymously with neo-Darwinism are modern synthesis, evolutionary synthesis, or neo-Darwinian synthesis.
Neo-Darwinism has been one of the most significant, overall developments in evolutionary biology since the time of Darwin. Bowler (1988) stated that there is "a sense in which the emergence of the modern synthetic theory can be seen as the first real triumph of Darwinism."
Essentially, neo-Darwinism introduced the connection between two important discoveries: the units of evolution (genes) with the mechanism of evolution (natural selection). By melding classical Darwinism with the rediscovered Mendelian genetics, neo-Darwinism recast Darwin's ideas in terms of changes in allele frequencies. It thus fused two very different and formerly divided research traditions, the Darwinian naturalists and the experimental geneticists. This fusion took place roughly between 1936 and 1947.
While the modern synthesis remains the prevailing paradigm of evolutionary biology, in recent years it has both been expanded and challenged as a result of new developments in evolutionary theory. In particular, concepts related to gradualism, speciation, natural selection, and extrapolating macroevolutionary trends from microevolutionary trends have been challenged.
Major figures in the development of the modern synthesis include Thomas Hunt Morgan, Ronald Fisher, Theodosius Dobzhansky, J.B.S. Haldane, Sewall Wright, William D. Hamilton, Cyril Darlington, Sergei Chetverikov, E. B. Ford, Julian Huxley, Ernst Mayr, George Gaylord Simpson, and G. Ledyard Stebbins.
Origin of the term neo-Darwinism
Originally, the term neo-Darwinism signified something quite different than it does today.
From the time of Darwin until the late 19th century, the term Darwinism had come to mean any of a wide diversity of views, including various social philosophies. Some of the views encased by the term were not centered around natural selection at all.
Near the end of the 19th century, one of the strong debates was between those who were promoting the inheritance of acquired characteristics (Lamarckism) and those who were promoting the exclusivity of natural selection. Prominent in this later camp were August Weismann and Alfred Russel Wallace. (Darwin himself was more pluralistic, in that he also found a place in his theory for the inheritance of acquired characteristics.)
In 1896, George John Romanes coined the term "neodarwinism" to designate the type of Darwinism being advanced by August Weismann. Weismann argued that hereditary material, which he called the germ plasm, was kept utterly separate from the development of the organism. He maintained that natural selection was the sole creative agent in evolution, and gave no credence to the inheritance of natural selection. This became the meaning of neo-Darwinism, and Weisman was the most prominent "neo-Darwinian." (This was seen by most biologists as an extreme position, however, and variations of neo-Lamarckism, orthogenesis ("progressive" evolution), and saltationism (evolution by "jumps" or mutations) were discussed as alternatives.)
However, the term neo-Darwinism was not terribly popular in the scientific community. It became popular only after the development of the modern evolutionary synthesis in the 1930s, when the term became synonomyous with the synthesis. The modern meaning of neo-Darwinism is not "genealogically linked" to the earlier use (Gould 2002).
History of the modern synthesis
Originally, very different research disciplines, working independently. Among the several branches of biology that previously had little in common were genetics, cytology, systematics, botany and paleontology.
In 1900, Mendelian inheritance was "rediscovered", and was initially seen as supporting a form of "jumping" evolution. The biometric school, led by Karl Pearson and Walter Frank Raphael Weldon, argued against it vigorously, stating empirical evidence indicated that variation was continuous in most organisms. The Mendelian school, led by William Bateson, countered that in some cases the Mendelian evidence was indisputable and that future work would reveal its larger truth. Mendelism was taken up by many biologists, even though it was still extremely crude at this early stage. Its relevance to evolution was still hotly debated. Mendelian genetics was rediscovered in 1900. However, there were differences of opinion as to what was the variation that natural selection acted upon. The biometric school, led by Karl Pearson followed Darwin's idea that small differences were important for evolution. The Mendelian school, led by William Bateson, however, thought that Mendel's work gave an evolutionary mechanism with large differences.
A critical link between experimental biology and evolution, as well as between Mendelian genetics, natural selection, and the chromosome theory of inheritance, arose from T. H. Morgan's work with the fruit fly Drosophila melanogaster. In 1910, Morgan discovered a mutant fly with solid white eyes (wild-type Drosophila have red eyes), and found that this condition—though appearing only in males—was inherited precisely as a Mendelian recessive trait. In the subsequent years, he and his colleagues developed the Mendelian-Chromosome theory of inheritance and Morgan and his colleagues published The Mechanism of Mendelan Inheritance in 1915. By that time, most biologists accepted that genes situated linearly on chromosomes were the primary mechanism of inheritance, although how this could be compatible with natural selection and gradual evolution remained unclear. Morgan's work was so popular that it is considered a hallmark of classical genetics.
This issue was partially resolved by Ronald Fisher, who in 1918 produced a paper entititled "The Correlation Between Relatives on the Supposition of Mendelian Inheritance", which showed using a model how continuous variation could be the result of the action of many discrete loci. This is sometimes regarded as the starting point of the synthesis, as Fisher was able to provide a rigorous statistical model for Mendelian inheritance, satisfying both the needs (and methods) of the biometric and Mendelian schools.
Morgan's student Theodosius Dobzhansky was the first to apply Morgan's chromosome theory and the mathematics of population genetics to natural populations of organisms, in particular Drosophila pseudoobscura. His 1937 work Genetics and the Origin of Species is usually considered the first mature work of neo-Darwinism. Mayr 1982) claimed that this wok "heralded the beginning of the synthesis, and in fact was more responsible for it than any other." Works by Ernst Mayr (systematics), G. G. Simpson (paleontology), G. Ledyard Stebbins (botany), C. D. Darlington (cytology) and Julian Huxley soon followed. Huxley coined both evolutionary synthesis and modern synthesis in his semi-popular work Evolution: The Modern Synthesis in 1942. Mayr felt that an international symposium at Princeton, New Jersey, January 2-4, 1947, marked the formal completion of the synthesis (Hull 1988; Mayr 1982).
More recent version of above paragraph (but above was edited by me):Morgan's student Theodosius Dobzhansky was the first to apply Morgan's chromosome theory and the mathematics of population genetics to natural populations of organisms, in particular Drosophila pseudoobscura. His 1937 work Genetics and the Origin of Species is usually considered the first mature work of neo-Darwinism, and works by Ernst Mayr (Systematics and the Origin of Species – systematics), G. G. Simpson (Tempo and Mode in Evolution – paleontology) , G. Ledyard Stebbins (Variation and Evolution in Plants – botany), these are considered the four canonical works of the modern synthesis. C. D. Darlington (cytology) and Julian Huxley also wrote on the topic; Huxley coined both evolutionary synthesis and modern synthesis
Tenets of Neo-Darwinism
According to Mayr, between 1936 and 1947, biologists of the diverse subdivisions of biology:
- "accepted two major conclusions: (1) that evolution is gradual, being explicatory in terms of small genetic changes and recombination and in terms of the ordering of this genetic variation by natural selection; and (2) that by introducing the populaiton concept...one can explain all evolutionary phenomena in a manner that is consistent both with the known genetic mechanisms and iwth the observational evidence of naturalists."
According to the modern synthesis as established in the 1930s and 1940s, genetic variation in populations arises by chance through mutation (this is now known to be sometimes caused by mistakes in DNA replication) and recombination (crossing over of homologous chromosomes during meiosis). Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift, gene flow and natural selection. Speciation occurs gradually when populations are reproductively isolated, e.g. by geographic barriers.
Though agreement is not universal on the parameters of the modern synthesis, many descriptions hold as basic the primacy of natural selection as the creative agent of evolutionary change, gradualism, and the extrapolation of microevolutionary process (changes within species) to macroevolutionary trends (changes about the species level, such as the origin of new designs and broad patterns in history). add.. mutaiton within structural genes is the source of variability in organis, and evolutionary change is shift of frequency of genes in a population. macrrodevotluionay tends come from gradual accumualation of small genetic changes.
Mayr (1963) notes that "the proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species." gould (1980) states it this way: "The core of this synthetic theory restates the two most characteristic assertions of Darwin himself: first, that evolution is a two-stage process (random variation as raw material, natural selection as a directing force); secondly, that evotuionary change is generally slow, steady, gradual, and continuous. . . Orthodox neo-Darwinians extrapolate these even and continuous changes to the most profound structural transitions in life.
It has been reported that the synthesis during the initial stages was more pluralistic, subdequently hardening into its later canonical formulations (Depew and Weber 1985; Gould 1982).
In a nutshell
The major tenets of the evolutionary synthesis, then, were that populations contain genetic variation that arises by random (ie. not adaptively directed) mutation and recombination; that populations evolve by changes in gene frequency brought about by random genetic drift, gene flow, and especially natural selection; that most adaptive genetic variants have individually slight phenotypic effects so that phenotypic changes are gradual (although some alleles with discrete effects may be advantageous, as in certain color polymorphisms); that diversification comes about by speciation, which normally entails the gradual evolution of reproductive isolation among populations; and that these processes, continued for sufficiently long, give rise to changes of such great magnitude as to warrant the designation of higher taxonomic levels (genera, families, and so forth)}}
+ ::— Futuyma, D.J. in Evolutionary Biology, Sinauer Associates, 1986; p.12
Challenges to Neo-Darwinism
The modern synthesis is the prevailing paradigm of evolutionary biology. However, it has been further expanded and even challened by a number of developments in evolutionary theory. IN particular, challenges have been postualed relative to the themes of gradualism, speciation, and natural selection, adn the policy of extrapolating microevolutionary trends from microevoltuioanry trends has also come under fire. Many of these chalenges are generally reformatulations.
go through the challenges
note the seriousness of the challenges and Gould's view
Further advances
The modern evolutionary synthesis continued to be developed and refined after the initial establishment in the 1930s and 1940s. The most notable paradigm shift was the so-called Williams revolution, after George C. Williams presented a gene-centric view of evolution in the 1960s. New version:The modern evolutionary synthesis continued to be developed and refined after the initial establishment in the 1930s and 1940s. The work of W. D. Hamilton, George C. Williams, John Maynard Smith and others led to the development of a gene-centric view of evolution in the 1960s.
The synthesis as it exists now has extended the scope of the Darwinian idea of natural selection, specifically to include subsequent scientific discoveries and concepts unknown to Darwin such as DNA and genetics that allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection, altruism, and speciation.
A particular interpretation of neo-Darwinism most commonly associated with Richard Dawkins asserts that the gene is the only true unit of selection. Dawkins further extended the Darwinian idea to include non-biological systems exhibiting the same type of selective behavior of the 'fittest' such as memes in culture.
See also: Population genetics
ReferencesISBN links support NWE through referral fees
- Dobzhansky, T. Genetics and the Origin of Species, Columbia University Press, 1937 ISBN 0-2310-5475-0
- Fisher, R. A. The Genetical Theory of Natural Selection, Clarendon Press, 1930 ISBN 0-1985-0440-3
- Futuyma, D.J. in Evolutionary Biology, Sinauer Associates, 1986; p.12
- Haldane, J. B. S. The Causes of Evolution, Longman, Green and Co., 1932; Princeton University Press reprint, ISBN 0-6910-2442-1
- Huxley, J. S., ed. The New Systematics, Oxford University Press, 1940 ISBN 0-4030-1786-6
- Huxley, J. S. Evolution: The Modern Synthesis, Allen and Unwin, 1942 ISBN 0-0284-6800-7
- Mayr, E. Systematics and the Origin of Species, Columbia University Press, 1942; Harvard University Press reprint ISBN 0-6748-6250-3
- Simpson, G. G. Tempo and Mode in Evolution, Columbia University Press, 1944 ISBN 0-2310-5847-0
- Wright, S. 1931. "Evolution in Mendelian populations". Genetics 16: 97-159.
- Mayr, E. and W. B. Provine, eds. The Evolutionary Synthesis: Perspectives on the Unification of Biology, Harvard University Press, 1980 ISBN 0-674-27226-9
- Allen, Garland. Thomas Hunt Morgan: The Man and His Science, Princeton University Press, 1978 ISBN 0691082006
- Dawkins, Richard. The Blind Watchmaker, W.W. Norton and Company, Reissue Edition 1996 ISBN 0-393-31570-3
- Smocovitis, V. Betty. Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology, Princeton University Press, 1996 ISBN 0-691-03343-9
| Basic topics in evolutionary biology | (edit) |
|---|---|
| Processes of evolution: evidence - macroevolution - microevolution - speciation | |
| Mechanisms: natural selection - genetic drift - gene flow - mutation - phenotypic plasticity | |
| Modes: anagenesis - catagenesis - cladogenesis | |
| History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis | |
| Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics |
de:Synthetische Evolutionstheorie et:Sünteetiline evolutsiooniteooria hu:Modern evolúciós szintézis nl:Moderne synthese sv:Modern evolutionär syntes
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