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17.2 Evolution as Genetic Change in Populations MRS. MACWILLIAMS ACADEMIC BIOLOGY Slide 2 I. How Natural Selection Works **Evolutionary fitness- success in passing genes to the next generation A.Natural Selection on Single-Gene Traits 1. Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution. Ex. Mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms. If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common. Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce. Slide 3 B.Natural Selection on Polygenic Traits **Remember, polygenic traits have a wide range of traits **The fitness of individuals may vary from one end of the curve to the other. **Natural selection can affect the range of phenotypes and hence the shape of the bell curve. Slide 4 1.Stabilizing Selection- individuals near the center of the curve have higher fitness than individuals at either end Slide 5 2.Directional Selection- individuals at one end of the curve have higher fitness than individuals in the middle or at the other end Slide 6 3.Disruptive Selection- individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Slide 7 Slide 8 Natural Selection and Adaptation (Rock Pocket Mouse) Watch Video at: http://www.hhmi.org/biointeractive/making-fittest-natural- selection-and-adaptation Slide 9 II. Genetic Drift ** In small populations, individuals that carry a particular allele may leave more descendants than other individuals, by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population. A.Genetic Bottlenecks- change in allele frequency following a dramatic reduction in the size of a population Ex. Disaster kills many individuals in a population, and surviving populations gene pool may contain different gene frequencies from the original gene pool. Slide 10 B. Founder Affect- allele frequencies change as a result of the migration of a small subgroup of a population Slide 11 III. Evolution Versus Genetic Equilibrium **Genetic Equilibrium- allele frequencies in the population remain the same. If allele frequencies dont change, the population will not evolve. A.Sexual Reproduction and Allele Frequency 1.Gene shuffling during sexual reproduction produces many different gene combinations 2.Meiosis and fertilization do not change allele frequencies by themselves, therefore a population of organisms could remain in genetic equilibrium Slide 12 B.Hardy Weinberg Principle- Describes conditions under which evolution does not occur **Allele frequencies remain constant unless one or more of the following 5 factors cause those frequencies to change 1.Large Population Genetic drift can cause changes in allele frequencies in small populations; Large population size helps maintain genetic equilibrium 2.No Mutations If mutations occur, new alleles may be introduced into the gene pool and allele frequencies will change Slide 13 3.Random mating All members of the population must have an equal opportunity to produce offspring and must mate with other members of the population at random In natural populations, mating is NOT random. Ex. Female peacocks choose mates on the basis of physical characteristics such as brightly patterned tail feathers alleles for those traits are under selection pressure Slide 14 4.No Gene Flow/Migration a. Individuals who join a population may introduce new alleles into the gene pool. b. Individuals who leave may remove alleles from the gene pool. c. Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population. Slide 15 5.No Natural selection a.All genotypes in the population must have equal probabilities of surviving and reproducing b.Phenotypes have NO selective advantage over another Slide 16 Should Antibiotic Use Be Restricted? Many disease-causing bacteria are evolving resistance to antibiotics. Bacterial populations have ALWAYS contained a FEW individuals with mutations that enable them to destroy, inactivate, or eliminate antibiotics. But those individuals didnt have a higher fitness, so mutant alleles didnt become common. Doctors began prescribing antibiotics widely and farmers started feeding them to their animals to prevent infection. Antibiotics have become a REGULAR PART OF THE ENVIRONMENT for the bacteria. In this environment, resistant bacteria have a higher fitness, so the resistance alleles increase in frequency. Resistant alleles can be transferred from one bacteria to another on plasmids. Many bacteria are evolving resistance not to just one antibiotic, but to almost ALL medicines! SHOULD ANTIBIOTIC USE BE RESTRICTED?