Lewontin and Hubby's paper also discussed the possible explanation of the high levels of variability by either balancing selection or neutral mutation. Although they did not commit themselves to advocating neutrality, this was the first clear statement of the neutral theory for levels of variability within species. Lewontin and Hubby's paper had great impact—the discovery of high levels of molecular variability gave population geneticists ample material to work on, and gave them access to variation at single loci. The possible theoretical explanations of this rampant polymorphism became the focus of most population genetics work thereafter. Martin Kreitman was later to do a pioneering survey of population-level variability in DNA sequences while a Ph.D. student in Lewontin's lab.