Theodosius Dobzhansky

Theodosius Grygorovych Dobzhansky (; ; January 25, 1900 – December 18, 1975) was a noted geneticist and an evolutionary biologist. 
Dobzhansky was born in Ukraine (then part of Imperial Russia) and emigrated to the United States in 1927. He was a central figure in the field of evolutionary biology for his work on the fruit-fly "Drosophila". He did research on these flies in California, mostly on populations of "Drosophila pseudoobscura". 
Dobzhansky was important in the modern evolutionary synthesis, which was the synthesis of evolutionary biology with genetics.
Ukraine and Russia.
At school Dobzhansky collected butterflies and beetles, and studied biology at Kiev University. After graduation he moved to St Petersburg (then called Leningrad), and studied under Yuri Filipchenko, who had a "Drosophila" lab. Dobzhansky collected Coccinellidae (ladybird beetles) in the wild, and explored their genetics.
America.
Dobzhansky emigrated to the United States in 1927. He worked with Thomas Hunt Morgan at Columbia University, who had pioneered the use of fruit flies ("Drosophila melanogaster") in genetics experiments. He followed Morgan to the California Institute of Technology from 1930 to 1940. Dobzhansky took fruit fly research out of the laboratory and into the field. He discovered that regional varieties of flies were more similar to each other genetically than to flies from other regions.
In 1937 Dobzhansky published one of the major works of the modern evolutionary synthesis, entitled "Genetics and the Origin of Species". He defined evolution as "a change in the allele frequency within a gene pool". It is through changes in the proportion of alleles in a population that evolution takes place. Also in 1937, he became a naturalized citizen of the United States. During this time he had a falling out with one of his "Drosophila" collaborators, Alfred Sturtevant, based perhaps on professional competition. 
Dobzhansky returned to Columbia University from 1940 to 1962. He was one of the signatories of the 1950 UNESCO statement "The Race Question". He then moved to the Rockefeller University) until his retirement in 1971.