Phenotypic Effects And Evolution
Mutations in the germ-line cells are heritable and provide the raw material upon which natural selection operates to produce evolution. Mutations in somatic cells, which are cells that are not germ line, are not heritable but may lead to disease in the organism possessing them.
Most mutations do not cause disease and are said to be "silent" mutations. This is for at least two reasons. First, most DNA does not code for genes, so changes in the sequence do not affect the types or amounts of protein made and there is no change in the phenotype of the organism. Second, most sexually reproducing organisms are diploid, meaning they possess two copies of every gene. Many types of mutation simply disable one copy, leaving the other intact and functional. Therefore these mutations display a recessive inheritance pattern, with no effect on phenotype unless an individual inherits two copies of the mutation. Diploid species can accumulate a large pool of such recessive mutations, which are mostly disadvantageous and thus contribute to the burden of genetic disease.
Some mutations lead to detrimental alterations of the normal pheno-type and are, therefore, selected against. Very occasionally, the mutant phenotype is superior and provides a selective advantage, which leads to an increase in the frequency of this mutant allele and, thus, to evolution of the population. Alternatively, a disadvantageous mutation in one environment may become advantageous in another, again leading to increased frequency of this allele.