Mutation

Mutation, change in the DNA of an organism (see Genetics: Gene Action: DNA and the Code of Life). Mutations occur as copying mistakes when the DNA is replicated, or as spontaneous changes within a DNA molecule. The rate of mutation can be increased by environmental mutagens, such as X-rays, but most mutations are internal errors within the organism, rather than externally caused. Mutations may take the form of changes to a nucleotide (a subunit of DNA); or may be larger-scale, in which a whole region of the DNA is inverted, deleted, doubled up, or moved elsewhere in the DNA. In some cases the whole DNA set of an organism may be doubled up (see Genetics: Chromosome Mutations). Some mutations can lead to the formation of a new species.

Mutations are the raw material for evolution (see also Variation). Evolution occurs when a new version of a gene, which originally arose by mutation, increases in frequency and spreads through the species by natural selection or random genetic drift (chance fluctuations in gene frequencies). Mutations were once thought to direct evolution, but the modern view is that mutations are undirected and that natural selection is the main directing force in evolution. Without mutation, however, species would not evolve.

Whether or not a mutation spreads depends on whether it improves the quality of the organism containing it (advantageous mutation); makes no difference to the organism (neutral mutation); or reduces the quality of the organism (disadvantageous mutation). Natural selection acts to increase the frequency of advantageous mutations, which is how evolutionary change occurs, because those organisms with advantageous mutations are more likely to survive, reproduce, and pass on their mutations to their offspring.

Natural selection acts to eliminate disadvantageous mutations and is therefore continually acting to protect a species from mutational decay. However, disadvantageous mutations arise as fast as natural selection eliminates them and populations are never completely cleared of inferior, mutant forms of genes. These disadvantageous mutations cause genetic diseases, and disadvantageous mutations that are recessive (see Genetics: The Transmission of Genes) cause inbreeding depression (decreased vigour in organisms that result from one or more generations of inbreeding).

Natural selection does not act on neutral mutations, but neutral mutations can change in frequency by random processes. There is controversy over the percentage of mutations that are neutral, but it is generally accepted that, of the non-neutral mutations, disadvantageous mutations are much more frequent than advantageous mutations. Natural selection therefore usually acts to reduce the mutation rate to the minimum possible, and observed mutation rates are indeed low.