Mutation generates the raw material for evolution

Individuals in populations may differ from one another in their phenotype, the observable characteristics of an organism, such as size or color (FIGURE 6.5). Many aspects of an organism's phenotype, including its physical features, metabolism, growth rate, susceptibility to disease, and behavior, are influenced by its genes.

FIGURE 6.5 Individuals in Populations Differ in Their Phenotypes Poisondartfrogs (Dendrobates tinctorius) show great variation in color and pattern. Native to northern South America, these frogs live in isolated patches of forest. Their bright colors are thought to serve as a warning to predators of a poison excreted from their skin. Individual frogs likely also differ in other morphological traits as well as in biochemical, behavioral, and physiological traits. View larger image

The formation of new alleles by mutation is critical to evolution.

Despite its importance to evolution, mutation usually occurs too rarely in most cases to be the direct cause of significant allele frequency change over short periods of time. Mutations typically occur at rates of 10^-4 to 10^-6 new mutations per gene per generation (Hartl and Clark 2007). In other words, in each generation, we can expect one mutation to occur in every 10,000 to 1,000,000 copies of a gene. At these rates, in one generation, mutation acting alone causes virtually no change in the allele frequencies of a population. Eventually, mutation can cause appreciable allele frequency change, but typically it takes thousands of generations for it to do so. Overall, in terms of its direct effects, the background mutation rate is a weak agent of allele frequency change. But because it provides new alleles on which natural selection and other mechanisms of evolution can act, mutation is central to the evolutionary process. It should be noted, however, that some environmental factors, such as exposure to high- energy radiation (e.g., radioactivity or X rays) and some mutagenic chemicals, can greatly increase mutation rates.

The evolution of antibiotic resistance is an example where mutation rates are frequent enough to influence allele frequencies in a population. There are around 40 trillion (4 ? 10¹³) bacterial cells in a human body, consisting of around 500 distinct species. Given the mutation rate described above, we could expect an appreciable number of alleles to appear in a population in each generation. The majority of these mutations are deleterious—that is, they lower growth and reproduction of the bacteria. However, some alleles confer greater resistance to antibiotics applied to kill them. As a result, the efficacy of antibiotics is potentially compromised, particularly with regular application, which enhances the potential for natural selection to favor the allele conferring resistance.