As we will see in Part Seven of this book, mutations are the raw material of evolution: they provide the genetic diversity that makes natural selection possible. This diversity can be beneficial in two ways. First, a mutation in a somatic cell may benefit the organism immediately. Second, a mutation in a germ line cell may have no immediate selective advantage to the organism, but it may cause a phenotypic change in the organism’s offspring. If the environment changes in a later generation, this mutation may be advantageous, and thus selected for, under the new conditions.

We noted that gene duplication can arise through either chromosomal rearrangements or through the movements of transposons. Gene duplication is not always harmful and is an important source of genetic variation. In a duplicated pair of genes, one gene may continue to play its original role in the cell, while the other may acquire a gain-of-function mutation that produces a new phenotype. As with any other mutation, this may be of immediate benefit for the organism, or it may provide a later generation with a selective advantage.

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By contrast, mutations in genes whose products are needed for normal cellular processes are often harmful, especially if they occur in germ line cells that produce eggs or sperm. In such cases, some offspring can inherit harmful recessive alleles in the homozygous condition. In their extreme form, such mutations produce phenotypes that are lethal, killing the organism during early development. Likewise, mutations in somatic cells can be harmful, as in the case of cancer. Think about what would happen if an oncogene that stimulates cell division in a colon cell had a gain of function mutation in an adult. The result would be the same as inheriting the mutant allele: colon cancer.

What can we do about mutation? While spontaneous mutagenesis is not in our control, we can certainly try to avoid mutagenic substances and radiation. Not surprisingly, many things that cause cancer (carcinogens) are also mutagens. A good example is benzopyrene (discussed above), which is found in coal tar, car exhaust fumes, and charbroiled foods, as well as in cigarette smoke. A major public-policy goal is to reduce the effects of both human-made and natural mutagens on human health. An example is the Montreal Protocol, an international environmental agreement signed and adhered to by all members of the United Nations. It bans chlorofluorocarbons and other substances that cause depletion of the ozone layer in the upper atmosphere of Earth. The thinning of the ozone layer can result in increased ultraviolet radiation reaching Earth’s surface, which is implicated in somatic mutations that lead to skin cancer.