 Sober (1980) raises a different objection to species essentialism. He illustrates how essentialist explanations have been replaced by evolutionary ones. Essentialists explain variation within a species as the result of interference in the ontogenetic development of particular members of that species. Organisms have species specific essences, but interference often prevents the manifestations of those essences. Contemporary geneticists offer a different explanation of variation within a species. They cite the gene frequencies of a species as well as the evolutionary forces that affect those frequencies. No species specific essences are posited. Contemporary biology can explain variation within a species without positing a species’ essence. So according to Sober, species essentialism has become theoretically superfluous.