Changes in the genetic code(DNA) are called mutations. They can occur spontaneously, or due to exposure to environmental agents. Mutations can only be passed down in gametes--sperm and egg cells. In the Darwin school of thought, a mutation needs to pass the test of natural selection to remain in the gene pool. An example is the sickle cell anemia mutation, a base-pair switch mutation. In this case, a person's hemoglobin folds into an irregular sickle shape. While a homozygote with sickle cell anemia doesn't have a survival advantage, a heterozygote with one sickle cell gene and one normal gene does! In countries where Malaria is prevalent, these "carriers" don't contract the dreaded Malaria--thus they have a selective advantage. These people live and reproduce, thus perpetuating the allele for sickle cell anemia in the gene pool. Usually, however, mutations are harmful and natural selection may weed them out of the population. A mutation usually results in a non-functional protein, or one that functions less well. However, any change is the DNA allows for evolution which means change over time. A mutation in the right environment can sometimes provide a selective advantage and over time, will be passed down to many offspring. Eventually, a new species may arise once enough changes accumulate.