The modern evolutionary synthesis (often referred to simply as the modern synthesis), neo-Darwinian synthesis or neo-Darwinism, brings together Charles Darwin's theory of the evolution of species by natural selection with Gregor Mendel's theory of genetics as the basis for biological inheritance. Major figures in the development of the modern synthesis include Ronald Fisher, Theodosius Dobzhansky, J.B.S. Haldane, Sewall Wright, Julian Huxley, Ernst Mayr, George Gaylord Simpson and G. Ledyard Stebbins. 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).
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.
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. This issue was finally 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 generally regarded as the starting point of the synthesis.
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 by geographic barriers.
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