Neo-Darwinism
Neo-Darwinism or the modern evolutionary synthesis (also referred to simply as modern synthesis, evolutionary synthesis, or neo-Darwinian synthesis) generally denotes the syntheis of Charles Darwin's theory of the evolution of species by natural selection, Gregor Mendel's theory of genetics as the basis for biological inheritance, and mathematical population genetics. This was not the historical meaning of neo-Darwinism, but has been the useage of the term since the synthesis of the 1930s. (See History.)
Neo-Darwinism has been the most significant, overall development in evolutionary biology since the time of Darwin. Bowler (1988) stated that "there is thus a sense in which the emergence of the modern synthetic theory can be seen as the first real triumph of Darwinism."
Essentially, the modern synthesis (or neo-Darwinism) introduced the connection between two important discoveries; the units of evolution (genes) with the mechanism of evolution (selection). By melding classical Darwinism with the rediscovered Mendelian genetics, neo-Darwinism recasted 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.
challenges to the modern synthesis
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.
History
Originally, the term neo-Darwinism signified something quite different. "To distinguish themselves from the very loose meaning of Darwinism prevalent in the 19th century, those who advocated evolution by natural selection after the death of Darwin became known as neo-Darwinists. The term "neodarwinism" itself was coined by George John Romanes in 1896 to designate the Darwinism proposed by August Weismann and Alfred Russel Wallace, in which the exclusivity of natural selection was promoted and the inheritance of acquired characteristics (Larmarckism) was rejected (Mayr 2001; Gould 2002). At that time, near the end of the 19th century, there was a strong debate between the neo-Larmarckians and the neo-Darwinians. The term neo-Darwinism was not terribly popular in the scientific community until 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 definition (Gould 2002).
George John Romanes coined the term neo-Darwinism to refer to the theory of evolution preferred by Alfred Russel Wallace et al. Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin, Huxley et al wouldn't rule out. The mechanism of inheritance wasn't discovered in Darwin or Wallace's time, however, so the debate was never settled.
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.
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 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.
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.
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).
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 due to 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.
Challenges to Neo-Darwinism
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. 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
- 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
| 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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