A third source of evolutionary evidence comes from the studies of genetics. According to evolutionary theory, only natural selection can provide direction for the evolution of a species. But genetic mutations and recombinations are sources of variability and add variety to a gene pool. Genetic variability is increased in a gene pool primarily by gene mutations. Random substitutions within DNA may correspond to a change in anatomy, physiology, and behavior. The altered traits often have an effect on the survival and reproduction of the individual. If this is the case and the mutation does not increase adaptive potential, natural selection will cause the traits to disappear from the population. Evolution at the genomic level is a feature which evolutionary theory uses to explain the emergence of new structures and new species.