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. 