 Mutations alter an organism 's genotype and occasionally this causes different phenotypes to appear . Most mutations have little effect on an organism 's phenotype , health , or reproductive fitness . Mutations that do have an effect are usually deleterious , but occasionally some can be beneficial . Studies in the fly Drosophila melanogaster suggest that if a mutation changes a protein produced by a gene , about 70 percent of these mutations will be harmful with the remainder being either neutral or weakly beneficial . 
