Modern evolutionary synthesis
The
modern evolutionary synthesis (often referred to simply as the
new synthesis, the
modern synthesis, the
evolutionary synthesis,
neo-Darwinian synthesis or
neo-Darwinism), generally denotes the integration 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, random genetic mutation as the source of variation, and mathematical
population genetics. Major figures in the development of the modern synthesis include
Thomas Hunt Morgan,
R. A. Fisher,
Theodosius Dobzhansky,
J.B.S. Haldane,
Sewall Wright,
William D. Hamilton,
Cyril Darlington,
Julian Huxley,
Ernst Mayr,
George Gaylord Simpson, and
G. Ledyard Stebbins.
Essentially, the modern synthesis introduced the connection between two important discoveries; the units of evolution (
genes) with the mechanism of evolution (
selection). It also represents a unification of several branches of biology that previously had little in common, particularly
genetics,
cytology,
systematics,
botany, and
paleontology.
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 most prominent "neo-Darwinian" of the time after Darwin was
August Weismann, who argued that hereditary material, which he called the
germ plasm, was kept utterly separate from the development of the organism. 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.
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.
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 conditionâ€"though appearing only in malesâ€"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 Mendelian 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
R. A. Fisher, who in
1918 produced a paper entitled
The Correlation Between Relatives on the Supposition of Mendelian Inheritance,
[Transactions of the Royal Society of Edinburgh, 52:399-433. ] 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 melanogaster. His 1937 work
Genetics and the Origin of Species is usually considered the first mature work of neo-Darwinism. This work, as well as works by Ernst Mayr (
Systematics and the Origin of Species – systematics), G. G. Simpson (
Tempo and Mode in Evolution – paleontology) , and G. Ledyard Stebbins (
Variation and Evolution in Plants – botany), 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 in his semi-popular work
Evolution: The Modern Synthesis in 1942.
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.
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.
"Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes."
[http://www.pnas.org/cgi/content/abstract/96/7/3801] Horizontal gene transfer is called by some "A New Paradigm for Biology "
[http://www.esalenctr.org/display/confpage.cfm?confid=10&pageid=105&pgtype=1] and emphasised by others as an important factor in "The Hidden Hazards of Genetic Engineering". "While horizontal gene transfer is well-known among bacteria, it is only within the past 10 years that its occurrence has becomerecognized among higher plants and animals. The scope for horizontal gene transfer is essentially the entire biosphere, with bacteria and viruses serving both as intermediaries for gene trafficking and as reservoirs for gene multiplication and recombination (the processof making new combinations of genetic material)."
[http://online.sfsu.edu/~rone/GEessays/horizgenetransfer.html] This approach is taken to its logical extreme by
Lynn Margulis in her theory of
symbiogenesis, with symbiosis as the major source of inherited variation, in which entire genomes are combined.
*
Gene-centered view of evolution*
Population genetics*
Symbiogenesis*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
*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
*Margulis, Lynn and Dorion Sagan. "Acquiring Genomes: A Theory of the Origins of Species", Perseus Books Group, 2002 ISBN 0-465-04391-7
*Mayr, E.
Systematics and the Origin of Species, Columbia University Press, 1942; Harvard University Press reprint ISBN 0-6748-6250-3
*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
*Simpson, G. G.
Tempo and Mode in Evolution, Columbia University Press, 1944 ISBN 0-2310-5847-0
*Smocovitis, V. Betty.
Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology, Princeton University Press, 1996 ISBN 0-691-03343-9
*Wright, S. 1931. "Evolution in Mendelian populations".
Genetics 16: 97-159.
