Bio 1B, Spring, 2008, Evolution section 1 of 4 Updated 3/6/08 1:54 PM Lectures 4-5 4-5 Mutation and selection Reading:7th edition 459-460, 464-468; 6th edition 452, 456-459. Mutation A mutation is the result of an error in DNA replication • A change in a single nucleotide is called a point mutation. In higher plants and animals, the probability of a point mutation (i. e. the mutation rate) is very low, close to 1 in a billion (10–9). Rates of point mutations are higher in bacteria and higher still in viruses. • Other types of mutations occur: small or large pieces of chromosomes can be deleted or duplicated. A duplication can create a second copy of a gene. • Whole chromosomes can be duplicated. In humans, individuals born with 3 copies of chromosome 21 (trisomy 21) have Down’s syndrome. The risk of trisomy 21 in the US population is between 1/650 and 1/1000; the risk increases with maternal age1. • Whole genomes can be duplicated. If there is no reduction division during meiosis, diploid gametes are produced. A diploid gamete combined with a haploid gamete creates a triploid zygote. Bananas and many other domesticated plants are triploid. Forces causing gene frequency change • Random mating does not cause allele frequencies to change, but other forces do. Mutation creates new alleles but mutation rates are so low that that mutation has little effect on the frequencies of alleles already present in a population. Alleles frequencies change because of the combined effects of natural selection, genetic drift, gene flow and recombination. Natural selection. Fitness • If individuals with different genotypes differ in their chances of survival and reproduction, then there are differences in fitness that cause allele frequencies to change. • Average fitnesses of different genotypes may depend on the environment. For example, individuals with defective alleles of the PAH gene who eat a normal diet die at an early age because of the accumulation of phenylalanine. They suffer from phenylketonuria (PKU). If phenylalanine is removed from the diet, there is almost no reduction in survival rate. • 1 Information about trisomy 21 and other inherited diseases and conditions in humans can be found from OMIM (Online Mendelian Inheritance in Man), http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM.Bio 1B, Spring, 2008, Evolution section 2 of 4 Updated 3/6/08 1:54 PM Lectures 4-5 Directional selection • Directional selection occurs when one allele results in higher rates of survival and reproduction. For example, if there are two alleles A and a, and AA individuals have a higher fitness than Aa individuals who have a higher fitness than aa individuals, then A is the advantageous allele and a is the deleterious allele. • If a population initially contains only aa individuals and an advantageous allele A is created by mutation, then the frequency of A will increase every generation because of natural selection. Eventually A will be substituted for a. • In the 1920s, population geneticists showed that natural selection can cause the substitution of an advantageous allele in a relatively few generations even if fitness differences are small. They concluded that even very weak natural selection acting of Mendelian alleles could cause changes in allele frequencies in a few hundred or a few thousand generations. Although that may be a long time from a human perspective, it is a short time compared to the long times available for evolution to occur. • An example of directional selection in humans is on alleles conferring lactose tolerance in adults. Of other mammals, only cats can digest lactose as adults. Most adults of European ancestry and some adults of east African ancestry can digest lactose because the gene coding for the enzyme lactase-phlorizin hydrolase (LPH) is expressed in adults. One mutation that enhances production of LPH in adults is responsible in Europeans and three different mutations are responsible in east Africans. All four mutations have reached high frequency in these populations because of directional selection in roughly the last 7000 years.2 • Past episodes of directional selection in humans may contribute to current health problems. Native Americans and some other groups are at a relatively high risk for type II diabetes (non-insulin-dependent diabetes) as adults. One explanation for this high risk is the “thrifty gene” hypothesis: ancestors of Native Americans experienced repeated famines that selected for efficient metabolism which causes type II diabetes in individuals who eat a modern diet. Some kinds of hypertension (high blood pressure) are caused by a tendency to retain too much salt, which possibly resulted from selection for salt retention during periods of low salt abundance. Although the thrifty-gene and salt-retention theories are plausible, they are not proved. Purifying selection • If a population contains only AA individuals and then a deleterious allele a is created by mutation, natural selection will tend to eliminate it. This is purifying selection. • Although mutation rates are very low, there are so many genes (more than 30,000 in humans) that deleterious alleles are created by mutation every generation. • A balance between new deleterious mutations and purifying selection acting to eliminate them will be reached. • For many genetic diseases in humans, including PKU, allele frequencies are determined by mutation opposed by purifying selection, called mutation-selection balance. • • 2 See http://www.nature.com/ng/journal/v39/n1/full/ng1946.html for details.Bio 1B, Spring, 2008, Evolution section 3 of 4 Updated 3/6/08 1:54 PM Lectures 4-5 Balancing selection • If heterozygous individuals have a higher fitness than either homozygote, selection will maintain both alleles in the population. The high frequency of the S allele of the β-globin gene in populations with a high incidence of malaria is explained by balancing selection. AA individuals are more likely to die of malaria as children than are AS individuals. As a consequence, S is maintained in frequencies as high as 12% in malarial regions even though SS individuals suffer from a severe disease, sickle cell anemia which often causes early death. • In non-malarial regions, S is deleterious and A is advantageous. S alleles are very rare in
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