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The modern synthesis
1930s–1940s· Theory or idea· Animals
Well establishedStrong fossil, genetic or geological evidence.
The 1930s–1940s merger of Darwin's natural selection with Mendel's genetics into a single theory of evolution.
See it on the Life map →What happened
After 1900 many biologists thought Mendel's genetics contradicted Darwin: new traits seemed to arise in sudden jumps, not through gradual selection. Mathematicians Ronald Fisher, J.B.S. Haldane and Sewall Wright showed that selection acting on many small genetic differences produces gradual change. Theodosius Dobzhansky, Ernst Mayr, George Gaylord Simpson and others then showed this fit field studies, the formation of species and the fossil record. Julian Huxley named it in his 1942 book Evolution: The Modern Synthesis.
Why it matters
It created the framework biologists still use: evolution as changing gene frequencies in populations, driven by selection, mutation, random drift and migration.
The evidence (fossils, genes, rocks)
Founding books by Fisher (1930), Dobzhansky (1937), Mayr (1942) and Simpson (1944); later DNA studies confirmed its core claims and added detail.
Still debated
Some biologists call for an 'extended synthesis' giving more weight to development, epigenetics and organisms reshaping their surroundings; others say the existing theory already covers these.
Connections (2)
▸Evolution by natural selection was merged with genetics in The modern synthesisWell established
The connection
By 1900 many biologists doubted natural selection, a period later called the 'eclipse of Darwinism', and rediscovered Mendelian genetics seemed to favor sudden jumps. From 1918, Fisher, Haldane and Wright showed mathematically that selection acting on many small Mendelian differences produces Darwin's gradual change, putting selection back at the center of evolution.
When and how
Britain and the U.S., from Fisher's 1918 paper on inheritance and continuous variation to the 1940s; the name came from Julian Huxley's 1942 book.
The evidence
The mathematics of Fisher's 1930 book and its match to field and lab studies, such as Dobzhansky's work on wild fruit fly populations.
Debates and open questions
How much evolution is driven by neutral drift rather than selection, a question sharpened by Motoo Kimura's neutral theory in 1968.
▸Genes & DNA: the science of heredity supplied the missing piece of The modern synthesisWell established
The connection
Darwin's theory lacked a working account of heredity: if parents' traits blended, any new variation would be diluted away. Mendel's genes do not blend; they pass on intact, keeping variation available for selection. The 1908 Hardy–Weinberg rule and later population genetics turned this into equations for how gene frequencies change.
When and how
Mendel rediscovered in 1900; G.H. Hardy and Wilhelm Weinberg's rule in 1908; population genetics by Fisher, Haldane and Wright, 1918–1932.
The evidence
Population-genetics predictions match measured changes in real populations, from lab fruit flies to wild peppered moths.
Debates and open questions
The synthesis was built before anyone knew genes were DNA; how far molecular discoveries have changed it is still argued.