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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, Julian Huxley, Ernst Mayr, Bernhard Rensch, George Gaylord Simpson, and G. Ledyard Stebbins.
Essentially, the modern synthesis introduced the connection between two important discoveries: the units of evolution (genes) and 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. Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin and Huxley 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 vigorously against it, stating that 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.
T. H. Morgan's work with the fruit fly Drosophila melanogaster provided critical evidence for the linkage of experimental biology and evolution, as well as that between Mendelian genetics, natural selection, and the chromosome theory of inheritance. 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 subsequent years, Morgan and his colleagues developed the Mendelian-Chromosome theory of inheritance, publishing 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,[1] which used a model to show how continuous variation could be the result of the action of many discrete genetic 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 the needs and methods of both 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 the terms 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, for example 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 to include subsequent scientific discoveries and concepts unknown to Darwin, such as DNA and genetics, which 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.