Evolution is the theory that a species will adapt over generations to improve its chances of survival in a changing world. Some geneticists believe that evolution in the human species may be coming to an end. According to Professor Steve Jones, males are more likely to pass on gene mutations after the age of 35 but modern society has children being born to fathers far younger. This means the offspring are not receiving gene mutations and are therefore not introducing significant change into the gene pool. Additionally, modern medicine has progressed such that while certain gene attributes probably were not fittest and should have died out, they are perpetuated anyway. Thus, humans are no longer improving themselves genetically but enhancing the overall weakening of natural survivability.