Ivan Ivanovich Schmalhausen (Russian: Ива́н Ива́нович Шмальга́узен; 23 April 1884 – 7 October 1963) was a Russian and Soviet zoologist and evolutionary biologist of German descent. He developed the theory of stabilizing selection, and took part in the development of the modern evolutionary synthesis.
He is remembered, among other things, for Schmalhausen's law, which states that a population at its limit of tolerance in one aspect is vulnerable to small differences in any other aspect.
The modern synthesis was the early 20th-century synthesis of Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity into a joint mathematical framework. Julian Huxley coined the term in his 1942 book, Evolution: The Modern Synthesis. The synthesis combined the ideas of natural selection, Mendelian genetics, and population genetics. It also related the broad-scale macroevolution seen by palaeontologists to the small-scale microevolution of local populations.
The synthesis was defined differently by its founders, with Ernst Mayr in 1959, G. Ledyard Stebbins in 1966, and Theodosius Dobzhansky in 1974 offering differing basic postulates, though they all include natural selection, working on heritable variation supplied by mutation. Other major figures in the synthesis included E. B. Ford, Bernhard Rensch, Ivan Schmalhausen, and George Gaylord Simpson. An early event in the modern synthesis was R. A. Fisher's 1918 paper on mathematical population genetics, though William Bateson, and separately Udny Yule, had already started to show how Mendelian genetics could work in evolution in 1902.
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