CH 16: Evolution of Population 16.1 Genes and Variation A. Introduction 1. Darwin s theory of evolution by natural selection explained how 2. What Darwin did not know was how were passed down through each generation 3. The study of genetics helps scientists understand the relationship between B. Variation and Gene Pools 1. Genetics a. Scientists know that and that many genes have at least two forms, or alleles. b. Also know that members of 2. In genetic terms, evolution is any change in the relative frequency of alleles in a population 3. Genetic variation is studied in populations. 4. - a group of individuals of the same species that interbreed. 5. 6. - all the genes, and their alleles, in the population. 7. - the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. 8. Relative frequency is often expressed as a. C. Sources of Genetic Variation 1. The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. a. - any change in a sequence of DNA. i. Mutations occur because of mistakes in DNA replication or as a result of. ii. Mutations do not always affect an organism s phenotype. b. - occurs during gamete formation i. Most differences are due to gene shuffling. ii. increases the number of genotypes that can appear in offspring. iii. Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population- Baby Face Lab D. Single-Gene and Polygenic Traits 1. The number of phenotypes produced for a given trait depends on how many genes control the trait. 2. Single-gene trait: is a trait controlled by a. If there are for the gene, two or three genotypes are possible. b. Example: the allele for a widow s peak is dominant over the allele for a hairline with no peak. i. So there are only 2 phenotypes - ii. However, the presence of a widow s peak may be less common in a population. iii. In real populations, phenotypic ratios are determined by the frequency of alleles as well as by whether the alleles are 3. Polygenic trait: is controlled by. a. One polygenic trait can have many possible genotypes and phenotypes. b. Example: polygenic trait.
c. Variation in a polygenic trait in a population often forms a bell-shaped curve with most members in the middle. d. A bell-shaped curve is also called normal distribution Polygenic Traits Follow the prompts to make a graph showing the frequency of different heights in a group of students Draw 1 bar for each height range. The bar should show how many students have heights in that range. Draw a curve connecting the tops of the bars 1. What shape is the curve you drew? ANSWER: 16-2 Evolution as Genetic Change A. Introduction 1. Natural selection affects which. 2. If an individual dies without reproducing, it 3. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. 4. Evolution is in the relative frequencies of alleles in a population. 5. B. Natural Selection on Single-Gene Traits 1. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. 2. Organisms of one color may produce fewer offspring than organisms of other colors. 3. Example: a lizard population is normally brown, but has mutations that produce red and black forms. a., so they will be less likely to survive and reproduce. Therefore, the allele for red color will become rare.
b.. This may give them energy to avoid predators. In turn, they may produce more offspring. c. The allele for black color will increase in relative frequency. 4. Natural selection can affect the distributions of in any of three ways: selection selection selection Natural Selection on a Single-gene Trait A colorful mutation occurred in a brown mouse population, causing darker fur. The table shows how the population changed over the next 30 generations. **What is happening to the relative frequency of the: a. Lighter fur color allele? b. Darker fur color allele? **Is the darker mutation favorable or unfavorable? C. Directional Selection 1. When individuals at one end of the curve have, directional selection takes place. 2. The range of phenotypes shifts as some individuals survive and reproduce while others do not. 3. Directional selection causes a 4. In this case, birds with larger beaks have higher fitness. Therefore, the average beak size increases D. Stabilizing Selection 1. When individuals near the individuals at either end of the curve, stabilizing selection takes place. 2. This keeps the center of the curve at its current position, but it the overall graph.(ex: Bell curve) 3. are more likely to survive than babies born either much smaller or much larger than average Ave. birth weight- 7lbs E. Disruptive Selection (sometimes called selection) 1. When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. 2. If the pressure of natural selection is strong enough and long enough, the curve. 3. If average-sized seeds become scarce, a bird population will split into two groups: one that eats small seeds and one that eats large seeds. F. Genetic Drift Directional selection graph Stabilizing selection graph Disruptive selection graph
1. A is called genetic drift 2. In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. 3. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. 4. Genetic drift may occur when a. 5. Individuals may carry alleles in different relative frequencies than did the larger population from which they came. 6. The new population will be. 7. When allele frequencies change due to migration of a small subgroup of a population it is known as the. G. Evolution Versus Genetic Equilibrium 1. To understand how evolution occurs, scientists first ask, Under? the Hardy-Weinberg principle answers this question. 2. The states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. 3. - the condition where allele frequencies remain constant 4. conditions are required to maintain genetic equilibrium from generation to generation: a. i. Random mating ensures that each individual has an equal chance of passing on its alleles to offspring. ii. In natural populations,. iii. Many species select mates based on particular heritable traits. b. i. Genetic drift has. ii. Allele frequencies of large populations are less likely to be changed through the process of genetic drift c. i. Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. ii. The population's gene pool must be kept together and kept separate from the gene pools of other populations. d. i. If genes mutate, new alleles may be introduced into the population, and allele frequencies will change. e. i. All genotypes in the population must have equal probabilities of survival and reproduction. ii. No phenotype can have a selective advantage over another. iii. There can be no natural selection operating on the population. 5. If all five conditions are met, relative allele frequencies will not change and evolution. 16-3 The Process of Speciation A. Introduction 1. Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation. 2. - the formation of new species. 3. - a group of organisms that breed with one another and produce fertile offspring *What factors are involved in the formation of new species? 4. The gene pools of two populations must become separated for them to become new species. B. Isolating Mechanisms 1. As new species evolve,
. 2. When the members of, reproductive isolation has occurred. 3. Reproductive isolation can develop in a variety of ways, including: a. - occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. b. - occurs when two populations are separated by geographic barriers such as rivers or mountains. i. Geographic barriers do not guarantee the formation of new species. ii. If two formerly separated populations can still interbreed, they remain a single species. iii. Potential geographic barriers may separate certain types of organisms but not others. c. - occurs when two or more species reproduce at different times C. Testing Natural Selection in Nature 1. Studies showing natural selection in action involve descendants of the finches that Darwin observed in the Islands. 2. The finches Darwin saw were different, but he hypothesized that they had. 3. Peter and Rosemary Grant tested Darwin s hypothesis, which relied on two testable assumptions:, there must be enough heritable variation in those traits to provide raw material for natural selection. Differences in beak size and shape, causing natural selection to occur. 4. The Grants tested these hypotheses on the medium ground finch on Daphne Major, one of the Galápagos Islands. 5. During the rainy season,. 6. During droughts,. 7. Individual birds with different-sized beaks had. 8. When food was scarce, individuals with. 9. The Grants provided evidence of the process of evolution. 10. 11. Describe the process of speciation in the Galápagos finches. D. Speciation in Darwin's Finches 1. Speciation in the Galápagos finches occurred by: a. - A few finches may have traveled from the mainland to one of the islands. There, survived and reproduced. b. - some birds then moved to a second island. The two populations were geographically isolated. They no longer shared a gene pool. c. - seed sizes on the second island favored birds with larger beaks. So this bird population evolved into a population with larger beaks. d. - in time, the large beaked birds were reproductively isolated fro birds on other islands and evolved into a new species e. - if birds from the second island cross back to the first, they live in competition. Individuals that are most different from one another compete less and are most able to reproduce. In time, this may lead to the evolution of yet another species. E. Studying Evolution Since Darwin 1. Scientific evidence supports the theory that. 2. Scientists predict that as, they will continue to expand our understanding of how species evolved.
3. Evolution continues today a. Example: Analyzing Data: Textbook p. 408 Complete #1-4 below. ANSWER: 1. 2. 3. 4.
Name Class Date Chapter 16 Evolution of Populations Graphic Organizer Flowchart In the following flowchart, arrange the events listed below in the order that they occur during speciation. Changes in the gene pool; Separation of populations; Continued evolution; Ecological competition; Founding of a new population; Reproductive isolation 1. 2. 3. 4. Pearson Education, Inc. All rights reserved. 5. 6. Teaching Resources /Chapter 16 203
Name Class Date Chapter 16 Evolution of Populations Chapter Vocabulary Review Completion On the lines provided, complete the following sentences. 1. The combined genetic information of all members of a particular population is called a(an). 2. The of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles occur. 3. A(an) is a trait controlled by a single gene. 4. are controlled by two or more genes. 5. The of an allele in a population is often represented by a percentage. Multiple Choice On the line provided, write the letter of the answer that best completes the sentence or answers the question. 6. For most genes, a gene pool typically contains a. no alleles. c. one allele. b. two or more alleles. d. no more than two alleles. 7. The graphs below show changes in the distribution of beak size in Galápagos finches during a period when food was scarce. What type of natural selection do the graphs show? a. directional selection c. stabilizing selection b. destabilizing selection d. disruptive selection Number of Birds in Population Beak Size Peak shifts; average beak size increases. Number of Birds in Population Beak Size 8. The graph below shows the distribution of human birth weights. What type of selection keeps this curve narrow and in the same place? a. directional selection c. stabilizing selection b. destabilizing selection d. disruptive selection Percentage of Population Birth Weight Selection against both extremes keeps curve narrow and in same place. Key Low mortality, high fitness High mortality, low fitness Pearson Education, Inc. All rights reserved. 200 Teaching Resources /Chapter 16
Name Class Date Pearson Education, Inc. All rights reserved. 9. What type of natural selection takes place when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle? a. directional selection c. stabilizing selection b. destabilizing selection d. disruptive selection 10. A random change in allele frequency is called a. fitness. c. speciation. b. genetic drift. d. the founder effect. 11. Genetic drift is more likely to occur in a. large populations. c. small populations. b. medium-sized populations. d. a single individual. 12. What occurs when allele frequencies change as a result of the migration of a small subgroup of a population? a. directional selection c. speciation b. the founder effect d. genetic equilibrium 13. What principle states that the frequency of an allele in a population will remain constant unless one or more factors cause that frequency to change? a. the speciation principle c. the Hardy-Weinberg principle b. the genetic equilibrium principle d. the genetic-drift principle 14. The situation in which allele frequencies do not change is called a. genetic equilibrium. c. behavioral equilibrium. b. stabilizing equilibrium. d. directional selection. 15. Which of the following is required to maintain genetic equilibrium? a. The population must be small. b. No mutations occur. c. Individuals move between populations. d. Natural selection occurs. 16. What is the formation of a new species called? a. directional selection c. founder effect b. speciation d. temporal isolation 17. What situation occurs when members of two different species cannot interbreed and produce fertile offspring? a. reproductive isolation c. genetic drift b. genetic equilibrium d. natural selection 18. What kind of isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior? a. courtship isolation c. geographic isolation b. behavioral isolation d. temporal isolation 19. Two populations kept separate by a river are characterized by a. genetic drift. c. geographic isolation. b. disruptive selection. d. temporal isolation. 20. What situation occurs when two or more species reproduce at different times? a. stabilizing selection c. geographic isolation b. behavioral isolation d. temporal isolation Teaching Resources /Chapter 16 201
Name Class Date Chapter 16 Evolution of Populations Enrichment A Close Look at Darwin s Finches When Charles Darwin traveled to the Galápagos Islands, he found a variety of species of finches. Although each species was slightly different from the others, all the species were related. None of the finch species he found were similar to finches on the mainland. When Darwin saw such extensive diversity of species in a single group of birds, he hypothesized that they all could have descended from a common ancestor. His observations of these finches helped him formulate his concept of evolution. The phylogenetic tree below shows the relationships Darwin proposed among the species of finches. The tree is based on a comparison of the anatomy, behavior, and location on the island of each finch species. Look carefully at each species, and notice the dramatic difference among the beaks. Each type of finch has a beak adapted to its diet. Darwin s finches are an example of adaptive radiation. Adaptive radiation is the emergence of many species from a common ancestor that was introduced to various new environments. For adaptive radiation to occur, the new environments must offer new opportunities and pose new problems of survival for the species. C. psittacula G. magnirostris C. pallidus C. pauper G. fuliginosa G. scandens Woodpecker-like Ground finches G. fortis Certhidea olivacea C. heliobates Insect-eating C. parvulus Cactus-eating G. conirostris C. crassirostris Vegetarian Camarhynchus (tree finches) Geospiza Ancestral seed-eating finch G. difficilis Warblerlike finches Pinaroloxias inornata Evaluation Answer the following questions on a separate sheet of paper. 1. Which of the ground finches illustrated above would be able to eat the largest, toughest nuts and seeds? Explain your answer. 2. Study the insect-eating finches shown in the diagram. What can you infer about the insects of the Galápagos Islands? Pearson Education, Inc. All rights reserved. 202 Teaching Resources /Chapter 16