Evolution of Populations

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1 Evolution of Populations Evolution Q: How can populations evolve to form new species? 17.1 How do genes make evolution possible? WHAT I KNOW SAMPLE ANSWER: There are different variations of the same gene. WHAT I LEARNED SAMPLE ANSWER: Evolution occurs when the allele frequency in the gene pool of a population changes over time What makes a population s gene pool change? SAMPLE ANSWER: Over time useful traits (and the genes that control them) accumulate in a population. SAMPLE ANSWER: Genetic changes can affect the number and types of possible phenotypes organisms in a population can have. These changes provide the variation that populations need to evolve How do new species form? SAMPLE ANSWER: The genome of a species changes enough that it becomes a new species. SAMPLE ANSWER: Speciation sometimes occurs when populations become reproductively isolated What can genes tell us about an organism s evolutionary history? SAMPLE ANSWER: Some species genomes are very similar. These species are closely related. The opposite is true for species with very different genomes. SAMPLE ANSWER: A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently. Chapter 17 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 267

2 17.1 Genes and Variation Lesson Objectives Define evolution in genetic terms. Identify the main sources of genetic variation in a population. State what determines the number of phenotypes for a trait. Lesson Summary Genetics Joins Evolutionary Theory Darwin s original ideas can now be understood in genetic terms. Researchers discovered that traits are controlled by genes and that many genes have at least two forms, or alleles. The combination of different alleles is an individual s genotype. Natural selection acts on phenotype, not genotype. Genetic variation and evolution are studied in populations. Members of a population share a common group of genes, called a gene pool. Allele frequency is the number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur. In genetic terms, evolution is any change in the allele frequency in a population. Sources of Genetic Variation The three main sources of genetic variation are mutations, genetic recombination during sexual reproduction, and lateral gene transfer. A mutation is any change in a sequence of DNA. Most heritable differences are due to genetic recombination during sexual reproduction. This occurs during meiosis when each chromosome in a pair moves independently. Genetic recombination also occurs during crossing-over in meiosis. Lateral gene transfer is the passing of genes from one organism to another organism that is not its offspring. Single-Gene and Polygenic Traits The number of different phenotypes for a given trait depends on how many genes control the trait. A single-gene trait is controlled by one gene. An example in snails is the presence or absence of dark bands on their shells. A polygenic trait is controlled by two or more genes, and each gene often has two or more alleles. An example of a human polygenic trait is height. Genetics Joins Evolutionary Theory For Questions 1 4, complete each statement by writing the correct word or words. 1. Natural selection works on an organism s phenotype rather than its genotype. 2. A(n) gene pool consists of all the genes, including the alleles for each gene, that are present in a population. Lesson 17.1 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 268

3 3. A gene pool typically contains different alleles for each heritable trait. 4. The number of times that an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur is called the allele frequency of the population. Use the circle graph of a sample mouse population to answer Questions In the diagram below, use circles to represent the alleles within each segment of the population. Draw the B alleles as solid circles and the b alleles as outline circles. The total number of individuals in this population is 25 ; the total number of alleles is 50. Sample Population 12 individuals: heterozygous black (Bb) individuals: homozygous black (BB) 9 individuals: homozygous brown (bb) How many alleles for black fur are in the sample population and what percentage of allele frequency does that represent? 20 B alleles, 40 percent 7. How many alleles for brown fur are in the sample population and what percentage of allele frequency does that represent? 30 b alleles, 60 percent 8. Describe how a geneticist might be able to tell that this population is evolving. The frequency of alleles will change. 9. Can you determine whether an allele is dominant or recessive on the basis of the ratio of phenotypes in the population? Explain your answer. No, because the phenotypic ratio depends on the allele frequencies of the dominant and recessive alleles, and the frequency of alleles has nothing to do with whether the allele is dominant or recessive. Sources of Genetic Variation 10. What are mutations? When do they affect evolution? A mutation is any change in the genetic material of a cell. Mutations only affect evolution when they occur in germ line cells that produce eggs or sperm and if they produce a change in phenotype that affects fitness. Lesson 17.1 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 269

4 11. How does sexual reproduction affect a population s genetic variation? Genetic recombination during sexual reproduction can produce many different phenotypes through the production of new and unique genetic combinations. 12. Identify two ways in which genes can be recombined during meiosis. by independent assortment of chromosomes and by gene swapping during meiosis 13. What is lateral gene transfer? How does it affect variation? Lateral gene transfer occurs when genes are passed from one organism to another organism that is not its offspring. It can occur between organisms of the same or different species. Lateral gene transfer increases variation when a species picks up new genes from a different species. Single Gene and Polygenic Traits 14. Label the two graphs to show which represents a single-gene trait and which represents a polygenic trait. Frequency of Phenotype Phenotype (height) Relative Frequency of Phenotype (%) Without bands With bands Phenotype Polygenic Trait Single-Gene Trait For Questions 15 19, write True if the statement is true. If the statement is false, change the underlined word or words to make the statement true. True polygenic 15. The number of phenotypes produced for a given trait depends on how many genes control the trait. 16. Height in humans is an example of a single-gene trait. Lesson 17.1 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 270

5 alleles True 17. Each gene of a polygenic trait often has two or more phenotypes. 18. A single polygenic trait often has many possible genotypes. True 19. A symmetrical bell-shaped graph is typical of polygenic traits. 20. Use the Venn diagram to compare and contrast single-gene traits and polygenic traits. Single-Gene Traits Both Polygenic Traits Controlled by only one gene; may only have two or three distinct phenotypes Controlled by genes Controlled by two or more genes; may have many phenotypes that are not clearly distinct from one another 21. Why is genetic variation important to the process of evolution? Genetic variation is the raw material of evolution, which can lead to different members of a population having different levels of fitness in a certain environment. The variation allows species to adapt to changes in their environment. Without such variation, the population would not evolve. Lesson 17.1 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 271

6 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 popula tions, alleles can become more or less com mon 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. Lesson 17.2 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 272

7 How Natural Selection Works 1. If a trait made an organism less likely to survive and reproduce, what would happen to the allele for that trait? Fewer copies of the allele would pass to future generations and the allele could even disappear from the gene pool completely. 2. If a trait had no effect on an organism s fitness, what would likely happen to the allele for that trait? The allele would not be under pressure from natural selection, and its frequency would probably stay about the same. Use the table showing the evolution of a population of mice to answer Questions 3 5. Initial Population Generation 10 Generation 20 Generation 30 90% 80% 70% 40% 10% 20% 30% 60% 3. Is the trait for fur color a single-gene trait or a polygenic trait? Explain your answer. The fur color is controlled by a single gene. There are only two phenotypes for this trait, gray or black fur. 4. Describe how the relative frequency of fur color alleles is changing in this population and propose one explanation for this change. The lighter fur color allele is decreasing in frequency and the darker fur color allele is increasing in frequency. Darker mice may be harder for predators to see, so they are more likely to survive and reproduce. 5. Suppose a mutation causes a white fur phenotype to emerge in the population. What might happen to the mouse population after 40 generations? SAMPLE ANSWER: If individuals with the new phenotype are more fit than the gray or black mice, the white allele may increase in frequency in the population. Black mice will likely continue to be more common than the other phenotypes. Lesson 17.2 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 273

8 6. What effect does stabilizing selection have on variation in a population? Stabilizing selection would generally reduce the variation in a population. For Questions 7 9, match the type of selection with the correct situation. Type of Selection B 7. Directional C 8. Stabilizing A 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. Draw the missing line in the graph on the right to show how disruptive selection affects beak size. Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Population splits into two subgroups specializing in different seeds. Number of Birds in Population Beak Size 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 allele frequencies 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 founder effect. 13. The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. Lesson 17.2 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 274

9 14. Complete the concept map. Genetic Drift can result from Bottleneck Effect Founder Effect caused dby caused dby a dramatic reduction in the size of a population the migration of a small subgroup of a population Evolution Versus Genetic Equilibrium 15. What does the Hardy-Weinberg principle state? Allele frequencies in a population should remain constant unless one or more factors cause them to change. 16. What is genetic equilibrium? the situation in which allele frequencies remain constant 17. List the five conditions that can disturb genetic equilibrium and cause evolution to occur. non-random mating, small population size, immigration or emigration, mutations, and natural selection 18. Explain how sexual selection results in non-random mating. When an individual practices sexual selection, or choosing a mate based on heritable characteristics such as size and strength, this individual s mate choice is not random. 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. In the original sandy habitat, tan insects may have been camouflaged from predators, making them more successful than green insects. When the habitat changes to a green, grassy meadow, individuals with green bodies may become more successful at hiding from predators. The green-bodied insects may survive and produce more offspring than the tan-bodied insects. Over time the frequency of the green-bodied allele would probably increase. Lesson 17.2 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 275

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