Dobzhansky’s book Genetics and the Origin of Species (1937) brought together the ideas of field naturalists and geneticists, both experimental and mathematical, to create a single argument on the process of evolution. His work with Drosophila had made him very aware that natural mutations can lead to large or small apparent changes, that these are acted upon by natural selection and that genes may interact and so give rise to variation which appears continuous rather than stepwise. He showed that natural selection could be seen in action, for example in the spread of scale insects resistant to control by cyanide gas, used as an insecticide in Californian citrus groves.

Dobzhansky developed good arguments for his view that a new species cannot arise from a single mutation and that, for a new species to form, it must for a period be isolated to protect it from disruption. The isolation could be geographical or due to differences in habitat or in the breeding season.

His work on human evolution led him to define races as ‘Mendelian populations which differ in gene frequencies’, as outlined in his Genetics of the Evolutionary Process (1970).