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Name Class Date 17.2 Evolution as Genetic Change in Populations Lesson Objectives Explain how natural selection affects single-gene and polygenic traits. Describe genetic drift. Explain how different factors affect genetic equilibrium. Lesson Summary How Natural Selection Works Natural selection on a single-gene trait can lead to changes in allele frequencies and changes in phenotype frequencies. For polygenic traits, populations often exhibit a range of phenotypes for a trait. When graphed, this range usually forms a bell curve, with fewer individuals exhibiting the extreme phenotypes than those with the average (in the case of beak size, the extremes may be tiny and large beaks). Natural selection on polygenic traits can cause shifts to the bell curve depending upon which phenotype is selected for. Directional selection takes place when individuals at one end of the bell curve have higher fitness than those near the middle or at the other end of the curve. For example, when large seeds are plentiful, large-beaked birds in a population may be selected for. Stabilizing selection takes place when individuals near the middle of the curve have higher fitness than individuals at either end. Disruptive selection takes place when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Genetic Drift In small populations, alleles can become more or less common simply by chance. This kind of change in allele frequency is called genetic drift. The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. The founder effect is a change in allele frequency that may occur when a few individuals from a population migrate to and colonize a new habitat. Evolution Versus Genetic Equilibrium If allele frequencies in a population do not change, the population is in genetic equilibrium. Evolution is not taking place. The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. These factors include: non-random mating, small population size, immigration or emigration, mutations, and natural selection. Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring. For example, many species exhibit non-random mating patterns. Sexual selection, or the process in which an individual chooses its mate based on heritable traits (such as size or strength), is a common practice for many organisms. 27 Name Class Date 6. What effect does stabilizing selection have on variation in a population? For Questions 7–9, match the type of selection with the correct situation. Type of Selection 7. Directional 8. Stabilizing 9. Disruptive Situation A. Individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. B. Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. C. Individuals near the center of the curve have higher fitness than individuals at either end. 10. THINK VISUALLY Draw the missing line in the graph on the right to show how disruptive selection affects beak size. Genetic Drift For Questions 11–13, complete each statement by writing the correct word or words. 11. In small populations, random changes in is called genetic drift. 12. A situation in which allele frequencies change as a result of the migration of a small subgroup of a population is known as the . 13. The is a change in allele frequency following a dramatic reduction in the size of a population. Name Class Date 14. Complete the concept map. Genetic Drift can result from Bottleneck Effect caused by caused by Evolution Versus Genetic Equilibrium 16. What is genetic equilibrium? 17. List the five conditions that can disturb genetic equilibrium and cause evolution to occur. Apply the Big idea 19. Suppose a population of insects live in a sandy habitat. Some of the insects have tan bodies and some have green bodies. Over time, the habitat changes to a grass-filled meadow. Use the ideas of natural selection to explain how and why the insect population might change. 275