Evolution as Genetic Change 16-2 PDF

16-2 Evolution as Genetic Change 81 7.a. Students know why natural selection acts on the phenotype rather than the genotype of an organism. *81 7.e. Students know the conditions for Hardy-Weinberg equilibrium in a population and why these conditions are not likely to appear in nature. *81 7.f. Stude...

16-2 Evolution as Genetic Change 81 7.a. Students know why natural selection acts on the phenotype rather than the genotype of an organism. *81 7.e. Students know the conditions for Hardy-Weinberg equilibrium in a population and why these conditions are not likely to appear in nature. *81 7.f. Students know how to solve the Hardy-Weinberg equation to predict the frequency of genotypes in a population, given the frequency of phenotypes. 81 S.c. Students know the effects of genetic drift on the diversity of organisms in a population. ·------Guide for Reading A genetic view of evolution offers a new way to look at key evolutionary concepts. Each time an organism reproduces, it passes copies of its genes to its offspring. We can therefore view evolutionary fitness as an organism's success in passing genes to the next generation. In the same way, we can view an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individual's ability to pass along its genes. Natural selection never acts directly on genes. Why? Because it is an entire organism-not a single gene-that either survives and reproduces or dies without reproducing. Natural selection, therefore, can only affect which individuals survive and reproduce and which do not. If an individual dies without reproducing, the individual does not contribute its alleles to the population's gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. Now recall that evolution is any change over time in the relative frequencies of alleles in a population. This reminds us that it is populations, not individual organisms, that can evolve over time. Let us see how this can happen in different situations. Key Concepts How does natural selection affect single-gene and polygenic traits? What is genetic drift? What is the Hardy-Weinberg principle? Vocabulary directional selection stabilizing selection disruptive selection genetic drift founder effect Hardy-Weinberg principle genetic equilibrium Reading Strategy: Outlining Before you read, use the headings to make an outline. As you read, add a sentence after each heading to provide key information. Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Imagine that a hypothetical population of lizards, shown in Figure 16-5, is normally brown, but experiences mutations that produce red and black forms. What happens to those new alleles? If red lizards are more visible to predators, they might be less likely to survive and reproduce, and the allele for red coloring might not become common. ~ Figure 16-5 ~ Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors. Effect of Color Mutations on Lizard Survival Initial Population Generation 10 Generation 20 Generation 30 ~~~~ ~~~~ ~~~~ ~~~~ ~~~~ ~~~ ~~ ~~ 80% 80% 70% 40% 0% 0% ~ 10% I 0% I I I ~ ~~ ~~~ ~~~ ~~~ 10% 20% 30% 60% J ~- - ' Evolution of Populations 397 Quick Lab Can the environment affect survival? Materials scissors, construction paper (several colors), transparent tape, 15-cm ruler, watch with a second hand Procedure ~~ 1. Choose three different-colored sheets of construction paper. Cut out a butterfly shape from each sheet, 5 x 10 em in size. CAUTION: Be careful with scissors. 2. Tape your butterflies to differentcolored surfaces. Then, return to your seat. 3. Record how many shapes of each color you can count from your desk in 5 seconds. 4. Exchange your observations with your classmates to determine the class total for each color. Analyze and Conclude 1. Analyzing Data According to your class data, which colors of butterfly are easiest to see? Which color of butterfly would be most easily caught by a predator? 2. Inferring What will happen to the butterfly population after many generations if predators consume most of the easy-tosee butterflies?.... Figure 16-6 ~ Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. In this example, a population of seed-eating birds experiences directional selection when a food shortage causes the supply of small seeds to run low. The dotted line shows the original distribution of beak sizes. The solid line shows how the distribution of beak sizes would change as a result of selection. 398 Chapter 16 Black lizards, on the other hand, might absorb more sunlight and warm up faster on cold days. If high body temperature allows them to move faster to feed and to avoid predators, they might produce more offspring than brown forms. The allele for black color might then increase in relative frequency. If a color change has no effect on fitness, the allele that produces it would not be under pressure from natural selection. Natural Selection on Polygenic Traits When traits are controlled by more than one gene, the effects of natural selection are more complex. As you learned earlier, the action of multiple alleles on traits such as height produces a range of phenotypes that often fit a bell curve. The fitness of individuals close to one another on the curve will not be very different. But fitness can vary a great deal from one end of such a curve to the other. And where fitness varies, natural selection can act. ~ Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection. Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. The range of phenotypes shifts as some individuals fail to survive and reproduce while others succeed. To understand this, consider how limited resources, such as food, can affect the long-term survival of individuals and the evolution of populations. Among seed-eating birds such as Darwin's finches, for example, birds with bigger, thicker beaks can feed more easily on larger, harder, thicker-shelled seeds. Suppose a food shortage causes the supply of small and medium-sized seeds to run low, leaving only larger seeds. Birds whose beaks enable them to open those larger seeds will have better access to food. Birds with the big-beak adaptation would therefore have higher fitness than small-beaked birds. The average beak size of the population would probably increase, as shown in Figure 16-6. Directional Selection Ill "C "· - s::::: a:l 0.....0 ·- Q) ::J +.J._.,!S!..ca. I E o ::Jc.. z.E / / I I / ,...-"' Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. This situation keeps the center of the curve at its current position, but it narrows the overall graph. As shown in Figure 16-7, the mass ofhuman infants at birth is under the influence of stabilizing selection. Human babies born much smaller than average are likely to be less healthy and thus less likely to survive. Babies that are much larger than average are likely to have difficulty being born. The fitness of these larger or smaller individuals is, therefore, lower than that of more average-sized individuals. Stabilizing Selection c: 0 += m "5 c. ~ 0 Cl) / C) m / Selection against both extremes keeps curve narrow and in ',same place. I_ c: I Cl) ~ ~ / / / / I Birth Mass._ Figure 16-7 In this example of stabilizing isruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. In such situations, selection acts most strongly against individuals of an intermediate type. If the pressure of natural selection is strong enough and lasts long enough, this situation can cause the single curve to split into two. In other words, selection creates two distinct phenotypes. For example, suppose a population of birds lives in an area where medium-sized seeds become less common and large and small seeds become more common. Birds with unusually small or large beaks would have higher fitness. As shown in Figure 16-8, the population might split into two subgroups: one that eats small seeds and one that eats large seeds. Genetic Drift In small populations, an allele can become more or less common simply by chance, rather than because it has positive or negative effects on fitness. The smaller a population is, the greater the chance that it will experience this kind of random change in allele frequency. This kind of random change in allele frequency is called genetic drift. How does genetic drift take place? ~ In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. selection, human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average. - ~~~ Disruptive Selection U) 'E mc: -o.Q._.!!! Cl) :::J.cO. E o :::JD.. z.~._ Figure 16-8 In this example of disruptive selection, average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds. Evolution of Populations 399 Sample of Original Population Descendants Founding Population A Go~nline active(ar, For: Genetic Drift activity Visit: PHSchool.com Web Code: cbp-5162 Figure 16-9 ~ In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group. Genetic drift may occur when a small group of individuals colonizes a new habitat. These individuals may carry alleles in different relative frequencies than did the larger population from which they came. If so, the population that they found will be genetically different from the parent population. Here, however, the cause is not natural selection but simply chancespecifically, the chance that particular alleles were in one or more of the founding individuals, as shown in Figure 16- 9. 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. One example of the founder effect is the evolution of several hundred species of fruit flies found on different Hawaiian Islands. All of those species descended from the same original mainland population. Those species in different habitats on different islands now have allele frequencies that are different from those of the original species. Hardy-Weinberg and Genetic Equilibrium To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place. So biologists ask: Are there any conditions under which evolution will not occur? Is there any way to recognize when that is the case? The answers to those questions are provided by the Hardy-Weinberg principle, named after two researchers who independently proposed it in 1908. The H a r dy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. The situation in which allele frequencies remain constant is called genetic equilibrium. If the allele frequencies do not change, the population will not evolve. 400 Chapter 16 Under what conditions does the HardyWeinberg principle hold? ~ Five conditions are required to maintain genetic equilibrium from generation to generation: (I) There must be random mating: (2) the population must be very large; and (3) there can be no movement into or out of the population, (4) no mutations, and (5) no natural selection. In some populations and in rare situations, these five conditions may be met or nearly met for long periods of time. If, however, the conditions are not met, the genetic equilibrium will be disrupted, and the population will evolve. Solving Problems Using HardyWeinberg It turns out that the Hardy-Weinberg principle is based on an equation that allows us to check its predictions. That equation can also be used to calculate and predict the frequency of certain genotypes. Imagine that you are a geneticist studying a trait controlled by two alleles, A and a. You know that these alleles follow rules of simple dominance. You survey a population for the trait, and discover that 4% of the population exhibits the phenotype produced by the homozygous recessive genotype aa. Fully 96% ofthe population isAA or Aa and exhibits the dominant phenotype. The Hardy-Weinberg equations represent the frequency of the dominant A allele asp and the frequency of the recessive a allele as q. The sum of the frequencies must always equal the entire population (100%). In mathematical form, this can be written as the equation: _. Figure 16-10 ~ One of the five conditions that are needed to maintain genetic equilibrium from one generation to the next is large population size. The allele frequencies of large populations, such as this group of birds, are less likely to be changed through the process of genetic drift. p+q=1 Recall from Chapter 11, that any cross that involves these alleles can produce three possible genotypes: AA, Aa, and aa. Now, when eggs and sperm are produced in members of this population, those gametes will carry these alleles in the same relative frequencies at which those alleles occur in the population. Thus, the relative frequency of eggs and sperm that carry the A allele will be equal top, and the relative frequency of eggs and sperm that carry the a allele will be equal to q. The three types of zygotes produced by these eggs and sperm will have the same relative numbers as the individuals in the Punnett square. Evolution of Populations 401 Those numbers can be expressed by the following equation: p2 + 2pq + q2 p2 = frequency of AA homo zygotes 2pq q2 =1 = the frequency of Aa heterozygotes = the frequency of aa homozygotes 1 = the sum of frequencies of all genotypes ( 100%) In a particular generation, we find that p = 0.8, and q = 0.2. How can you figure out the relative frequencies of AA, Aa, and aa individuals? 1. First, write the following equation: p2 + 2pq + q2 = 1 (or A2 + 2Aa + a2 = 1) 2. Fill in the values. (0.8)2 + 2(0.8 X 0.2) + (0.2)2 = 1 3. Calculate. (0.8 X 0.8) + 2(0.16) + (0.2 X 0.2) =1 0.64 + 0.32 + 0.04 = 1.00 4. Convert the fractions to percentages. 0.64 x 100 = 64%, so 64% is the frequency of homozygous dominant individuals (AA). 0.32 x 100 = 32%, so 32% is the frequency of heterozygous recessive individuals (Aa). 0.04 x 100 = 4%, so 4% is the frequency of homozygous recessive individuals (aa). As long as the Hardy-Weinberg equilibrium conditions hold, neither the frequency of the genotypes nor the frequencies of the alleles (p and q) will change from generation to generation. 1. ~Key Concept Describe how natural selection can affect traits controlled by single genes. 2. ~ Key Concept Describe three patterns of natural selection on polygenic traits. Which one leads to two distinct phenotypes? 3. ~ Key Concept How does genetic drift lead to a change in a population's gene pool? 402 Chapter 16 4. ~ Key Concept What is the Hardy-Weinberg principle? 5. Critical Thinking Calculating You are studying a population of 100 people and discover that 36 of these people are ss for a genetic condition. Use the HardyWeinberg equation to figure out the frequencies of the 5 and s alleles. What are the frequencies of the 55, 5s, and ss genotypes? Using Models Demonstrate natural selection on polygenic traits by cutting a sheet of paper into squares of five different sizes to represent sizes in a population. Use the squares to model directional, stabilizing, and disruptive selection. Name____________________________ Class __________________ Date __________ Natural Selection on a Single-Gene Trait A color mutation occurred in a brown mouse population, causing darker fur. The table below shows how the population changed over the next 30 generations. Initial Population Generation 10 Generation 20 Generation 30 90% 80% 70% 40% 20% 30% 60% 10% Use the table to answer the questions. 1. What is happening to the relative frequency of the lighter fur color allele? 2. What is happening to the relative frequency of the darker fur color allele? 3. Is the darker color mutation favorable or unfavorable? 4. What might cause the change shown in the table? 5. How do you predict the mouse population will look after 40 generations? © Pearson Education, Inc., publishing as Pearson Prentice Hall. 148 Name____________________________ Class __________________ Date __________ Number of Birds in Population Directional Selection A population of birds eats seeds. Small seeds can be eaten by birds with small beaks. Larger, thicker seeds can be eaten only by birds with larger, thicker beaks. Suppose there is a shortage of small seeds, but that there are still many large seeds. Draw a new curve on the graph to show how the distribution of beak sizes might change as a result of selection in this environment. original population Beak Size Use the graph to answer the questions. 1. Which birds in this population have the highest fitness? Circle the best answer. small-beaked birds large-beaked birds 2. Explain how natural selection could lead to the change you predicted. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 149 Name____________________________ Class __________________ Date __________ Number of Birds in Population Percentage of Human Population Stabilizing and Disruptive Selection In most populations, a trait that has higher fitness leads to greater numbers of organisms with that trait. On the graphs, dotted lines represent the original population. The solid lines represent the population after selection has taken place. Identify whether each graph shows stabilizing selection or disruptive selection. Write the type of selection shown below each graph. original population original population Beak Size Birth Weight Use the graphs to answer the questions. 1. Under which type of selection do organisms in the middle of the curve have the highest fitness? Circle the correct answer. disruptive stabilizing 2. In disruptive selection, organisms represented by which part of the curve have the lowest fitness? Circle the correct answer. middle of the curve ends of the curve 3. Describe a situation that might lead to the changes shown in the graph on the right. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 150 Name____________________________ Class __________________ Date __________ Genetic Drift In a small population, an individual with particular alleles may have more descendants than another individual, by chance. This kind of chance can, over time, lead to an allele’s becoming more common in a population. Draw what the descendants of these populations might look like. Draw 12 descendants for each population. Descendants Sample of Original Population Founding Population A Founding Population B Use the diagrams to answer the questions. 1. Draw a beetle that could be found in both descendant populations. 2. Why are the beetles in the two descendant populations different? © Pearson Education, Inc., publishing as Pearson Prentice Hall. 151