From Today In Science History:
Dobzhansky (Jan. 25, 1900 – Dec. 18, 1975) was an Ukrainian-American geneticist and evolutionist whose work had a major influence on 20th-century thought and research on genetics and evolutionary theory. He made the first significant synthesis of Charles Darwin's theory of evolution with Gregor Mendel's theory of genetics in his book Genetics and the Origin of Species (1937).
From 1918 his research gave experimental evidence that genes could vary far more than geneticists had previously believed. Thus, successful species tend to have a wide variety of genes that, while redundant in its present environment, do provide a species as a whole with genetic diversity. Such diversity enables the species to adapt effectively to changes in the surrounding environment - the basis for modern evolutionary theory.