Evolution of Populations PDF

9.1 Evolution of Populations CAN YOU EXPLAIN IT? These Asian beetles vary in Ruffs are birds found mostly in parts of Europe and Asia. They typically make their the color of their wings and...

9.1 Evolution of Populations CAN YOU EXPLAIN IT? These Asian beetles vary in Ruffs are birds found mostly in parts of Europe and Asia. They typically make their the color of their wings and in the number and color of homes in marshes and mudflats where they feed primarily on insects and seeds. spots on those wings. During breeding season, males gather in groups and participate in staged “fights” to attract females. In this courtship ritual, three types of males in the population are involved: the independents, the satellites, and the faeders. FIGURE 1: Ruffs differ in body size as well as in the size and color of the feathers on their heads and necks. Image Credits: (t) ©blickwinkel/Hecker/Alamy; (bl) (br) ©Arco Images GmbH/C. Wermter/Alamy; (bc) ©Matthijs Kuijpers/Alamy Gather Evidence As you explore the lesson, make a list of biotic or abiotic factors that may have contributed to the evolution of this population. a An independent male b A satellite male c A female. Faeder males greatly resemble females. About 84 percent of the male ruffs are “independent.” These ruffs fight hard and expend a lot of energy to establish a territory and attract female ruffs. They can be easily identified, as they are the largest males and have large black and brown neck feathers. “Satellite” males are smaller and have white neck feathers. They move freely between independents’ territories and do not fight. Though independent males may dominate them to attract a female, the satellites are often able to mate with the same females. The smallest males, called “faeders,” look similar to females and generally mate with females by sneaking, often when independents and satellites are distracted or fighting. Predict How can three types of males evolve in one population? 418 Unit 9 Patterns of Evolution  EXPLORATION 1 Genetic Variation Meerkats are mammals that live in the deserts of Africa. They live together in cooperative groups. FIGURE 2: Meerkats stand alert to look for predators. Gather Evidence Record the similarities and differences you see between the meerkats in Figure 2. Why do traits vary between individuals in a population? Differences in the Gene Pool As you looked at the physical traits, or phenotypes, of the meerkats, you might have Collaborate Meerkats noticed variations in some of their traits. For instance, some are smaller than others. have a range of fur colors, One has a light underside, while most have a darker underside. A few have more white from very light brown with more on their faces, and others have more brown. silver to a medium brown with less The phenotypic differences that you observed among the meerkats are due to silver. Imagine a plant species with differences in genes that code for those traits. Certain differences may offer a similar colors to the darker brown competitive advantage compared to the rest of the population. A particular phenotype meerkats starts to grow in their may allow individuals to survive longer and reproduce more efficiently, both of habitat. With a partner, discuss which increase the total number of offspring produced. So, over time the phenotype what would happen to the becomes more prevalent. This gradual favoring of advantageous traits within a meerkats and why. population is called natural selection, and it directly affects the population’s gene pool. A gene pool is the collection of alleles found in all of the individuals of a population. The different alleles in a gene pool ultimately result from mutations. When mutations occur during meiosis, the gametes that result may carry these mutations. Genetic Image Credits: ©EcoView/Fotolia variation may also be a result of crossing over and recombination, which occur during meiosis. During this process, chromosomes condense and homologous chromosomes align. Homologous chromosomes have the same genes but could have different Explain How can alleles. During the alignment, an exchange of genetic material may take place. This mutations in gametes exchange could alter the rearrangement of the linked genes in the chromosomes. As a become widespread in the gene result, the gametes are not genetically identical. pool? Lesson 1 Evolution of Populations 419  Variation in Alleles Different combinations of alleles in a gene pool can be formed when organisms mate and have offspring. Alleles are different forms, or versions, of genes. For example, mice with either one or two copies of the dominant B allele have brown fur, while mice with two recessive b alleles have black fur, shown in Figure 3. Gather Evidence FIGURE 3: Differences in fur color in mice are due to differences in allele combinations. Use the image to determine Bb BB how many total alleles, dominant BB alleles (B), and recessive alleles (b), are in the gene pool of this mouse population. bb bb BB Math Connection You can use the total number of alleles, the number of dominant alleles, and the number of recessive alleles to find the allele frequency in a population. Allele Use Figure 3 and the allele frequency is the proportion of one allele, compared with all the alleles for that trait, in frequency equation to answer the gene pool. To find the frequency of a particular allele, divide the number of times the following questions: the allele is present by the total number of alleles in the population. 1. What is the allele frequency of Number of particular allele the dominant allele B? Express Allele Frequency = ____________________ Total number of alleles your answer as a decimal Allele frequency can also be expressed as a percentage by multiplying the frequency rounded to the thousandths by 100. The frequencies of all the different alleles in a population should equal 1.0, or place and as a percentage. 100 percent. 2. What is the allele frequency of Allele frequency is used to track genetic variation in populations and detect changes the recessive allele b? Express in alleles. Imagine that periodic fires blacken the ground in the field mice habitat your answer as a decimal in Figure 3. The black mice may be better camouflaged, providing more protection rounded to the thousandths against predators. If they survive and reproduce more effectively than brown mice, place and as a percentage. the frequency of the b allele may increase over time relative to the B allele frequency. Analyzing Population Evolution Some chickens, ducks, and other birds can lay eggs that have either white or blue shells. Blue eggshells are dominant and are coded for by allele O. White eggshells are recessive and are coded for by allele o. The outcome of a heterozygous-heterozygous cross for eggshell color can be determined by creating a Punnett square. We can create a Punnett square to represent any dominant or recessive allele in a population for this type of cross. In this generic Punnett square, p represents any dominant allele and q represents any recessive allele. The Punnett square that gives the possible genotypes of the offspring of heterozygous parents for eggshell color is shown in Figure 4. 420 Unit 9 Patterns of Evolution The Punnett square shows that the genotypic frequency of 2 FIGURE 4: In this Punnett square, p represents any dominant OO is represented as p , Oo is represented as 2pq, and oo is 2 allele and q represents any recessive allele. represented as q. The frequency of all possible genotypes in a population must equal 1. If allele frequency can be found 2 2 O (p) o (q) using the equation p + q = 1, then p + 2pq + q = 1. Scientists use these equations to predict the genotypic frequencies in a population. Then, they compare the predicted frequencies to the actual frequencies in a population. O (p) OO (p 2) Oo (pq) Predict What could a scientist conclude if the genotypic frequencies in a population are different from the o (q) Oo (pq) oo (q 2) predicted values? Data Analysis In a population of 1,000 chickens, 840 hens lay blue eggs and 160 hens lay white VARIABLES 2 2 eggs. Use the equation p + 2pq + q = 1 to determine the predicted genotypic p = frequency of O frequencies for this population. Then compare those values with the actual (dominant allele, blue shell) genotypic frequencies in the population. q = frequency of o 2 S T E P 1 Solve for q by dividing the number of oo chickens by 1,000. (recessive allele, white shell) 2 160 2 p = frequency of chickens with OO q = = 0.16 1000 (homozygous dominant genotype) S T E P 2 Solve for q by taking the square root of each side of the equation. 2pq = frequency of chickens Oo (heterozygous genotype) q = 30.16 = 0.4 2 q = frequency of chickens with oo S T E P 3 Determine p by substituting the value of q in the equation p + q = 1: (homozygous recessive genotype) p + 0.4 = 1 p = 1 – 0.4 = 0.6 These are the predicted allele frequencies: p = 0.6 and q = 0.4. S T E P 4 Calculate the predicted genotypic frequencies from the predicted allele frequencies: 2 2 p = (0.6) = 0.36 2pq = 2 (0.6)(0.4) = 0.48 2 2 q = (0.4) = 0.16 Analyze Answer the following questions in your Evidence Notebook: 1. What percentage of this population is expected to be OO, Oo, and oo? What do these values mean? 2. Through genetic analysis, scientists discovered the actual genotypic frequencies for the above population to be OO = 0.60, Oo = 0.14, and oo = 0.26. What can you infer by comparing these data to the values predicted above? Lesson 1 Evolution of Populations 421  2 2 The equation p + 2pq + q = 1 is known as the Hardy-Weinberg equation. A Hardy- Weinberg population is in equilibrium, meaning it is stable and not evolving. Five conditions must be met for a population to be in equilibrium: no mutations, very large population, no natural selection, no new genetic material is introduced, and individuals are equally likely to mate with any other individual in the population. 1. v Cause and Effect FIGURE 5: Peppered Moths Selection on Peppered Moth Populations The peppered moth Biston betularia found in the English countryside, ranges in color from light (Biston betularia typica) to dark (Biston betularia carbonaria). Before the Industrial Revolution, light moths were more prevalent than dark moths. During the Industrial Revolution, trees became covered in dark soot from coal burned in factories. Over time, scientists observed that the number of dark moths increased relative to light moths. More recently, clean air laws returned the trees to their lighter coloring, and the dark colored moths decreased in frequency (Figure 6). Recent studies found bird predation was one possible driving force behind the population shift. When trees were covered with soot, birds preyed on light moths. When the soot faded, birds preyed on dark moths (Figure 7). Other factors, such as migration, may have also influenced the population and require further study. Analyze Create a graph of the shift observed in the peppered moth population. Place the color range on the x-axis and frequency of the trait on the y-axis. Frequency of dark moths around Leeds, Effect of bird predation on the population of England, from 1970-2000 light and dark moths FIGURE 6: Frequency of Dark Moths FIGURE 7: Effect of Bird Predation 1.0 1.0 typica variation Frequency of carbonaria moths 0.8 0.9 carbonaria variation Fraction surviving 0.6 0.8 0.7 0.4 0.6 0.2 0.5 0 2001 2002 2003 2004 2005 2006 2007 2008 Image Credits: (t)©Perennou Nuridsany/Science Sourcew 1960 1970 1980 1990 2000 2010 Year Year Source: Cook, L. M., B. S. Grant, and I. J. Saccheri, J. Mallet. "Selective bird predation Source: John N. Thompson, Relentless Evolution (2013): 57, quoted in on the peppered moth: the last experiment of Michael Majerus." Biol. Lett. 2012. http://phenomena.nationalgeographic.com/2013/10/09/ Published 8 February 2012. doi: 10.1098/rsbi.2011.1136. evolution-in-color-from-peppered-moths-to-walking-sticks/ Explain How does the Hardy-Weinberg equilibrium equation use genetic variation and allele frequencies in a population to describe whether a population is evolving? 422 Unit 9 Patterns of Evolution  EXPLORATION 2 Selection on Populations Though king penguins look similar, members of the population differ in some of their physical traits. Some penguins may be larger and some smaller. Some individuals may have long beaks, and some may have short beaks. The majority of penguins have characteristics somewhere between these two extremes. Normal Distribution If penguin beak lengths and their frequencies are graphed, the result is a bell-shaped curve, shown in Figure 8. The shape of the curve shows that the beak length of the majority of the individuals is close to the mean length. Mean (also called average) beak length is determined by adding the beak lengths of all the individuals and then dividing the sum by the number of Normal Distribution individuals. The graph also shows that there are not many FIGURE 8: Most individuals in this population have traits that individuals with extreme traits (very short or very long beaks). fall between two extreme phenotypes. mean Analyze Why do few individuals have very extreme phenotypes, such as very long or very short beaks, and more individuals show a trait somewhere in between? Frequency A normal distribution shows an arrangement of data in which most of the values fall in the middle of the data set, represented by the mean. The curve that results is bell-shaped and symmetrical. The frequency is highest near the mean value and decreases toward each extreme end of the range. This means that for a population showing normal distribution, the alleles for the mean phenotype are more advantageous Range of variable than the alleles associated with either extreme phenotype. Changing Populations King penguins live and breed on islands around Antarctica. FIGURE 9: King Penguins Like other penguin species that live in cold areas, king penguins have features that allow them to live in this type of environment. They have layers of feathers as well as thick layers of fat to help keep them warm. Suppose the climate in this area warms up and continues to warm up. How might this Image Credits: (b) ©Digital Vision/Getty Images continuing change in temperature affect the population? Collaborate Suppose as a result of increasing temperatures, the trait for having a thick layer of fat was selected against and the thinner layer of fat was selected for. With a partner, discuss how the normal distribution graph will be affected. Lesson 1 Evolution of Populations 423  In populations, natural selection favors phenotypes that allow individuals in the population to adapt to their environment and selects against phenotypes that make individuals less able to adapt to their environment. This “favoring” and “selecting against” result in observable changes in the allele frequencies in a population. Microevolution is the observable change in the allele frequencies of a population over time. Microevolution occurs on a small scale—within a single population. Stabilizing Selection In humans, very low or very high birth weight can cause complications that affect a baby’s health. Many infants with very low or very high birth weights do not survive to adulthood. Over many generations, these two phenotypes were selected against. More average birth weights, which had fewer weight-related Stabilizing Selection Explore Online complications, were selected for. Today, the frequency of individuals with an average birth weight is higher than those FIGURE 10: In stabilizing selection, intermediate phenotypes with extremely low or extremely high birth weights. are selected over phenotypes at both extremes. This type of selection is called stabilizing selection. This is the mean type of natural selection in which intermediate phenotypes are selected over phenotypes at both extremes. In the example of birth weight in humans, individuals with average birth weights were more successful than those with very low or very high birth weights. Frequency In stabilizing selection, extreme phenotypes are selected against. Over time, the survival rate of the individuals with these phenotypes decreases, so the frequency of these traits in the population also decreases. Phenotypes near the mean are selected for, so individuals that express these traits survive and reproduce more effectively than individuals without Range of variable these traits. This results in an increase in the frequency of these phenotypes in the population. Directional Selection Another type of selection can be seen in the case of the peppered moth. Recall that before the Industrial Revolution, there were more sightings of light-colored (typica) moths and few sightings of dark-colored (carbonaria) moths. As factories were built during the Industrial Revolution, pollution increased. At this time, scientists observed that the number of typica moths decreased, while the number of the carbonaria moths increased and became more abundant in the population than the typica variety. Model In your Evidence Notebook, draw a normal distribution graph for peppered moth coloration before the Industrial Revolution. Then, show how the frequencies of the phenotypes changed during the Industrial Revolution. The type of selection observed in peppered moths is called directional selection. This is the type of natural selection in which one extreme phenotype is selected over the other extreme phenotype, shifting the mean toward one of the extremes. In the case of the peppered moths, the dark phenotype was selected over the light phenotype during the Industrial Revolution. 424 Unit 9 Patterns of Evolution  Explore Online Directional Selection FIGURE 11: After directional selection occurs, an extreme phenotype becomes the more abundant phenotype. mean mean Frequency Range of variable In directional selection, one extreme phenotype becomes more advantageous in the environment. Over time, individuals with this trait are more successful than individuals without the trait. Directional selection shifts the phenotypic frequencies, favoring individuals with genotypes that code for the extreme phenotype. The mean value of the trait shifts in the direction of the more advantageous phenotype. Disruptive Selection Lazuli buntings are birds found in the western part of the United States. The male birds Analyze In your Evidence have feathers with colors that range from brown to bright blue. The dominant adult Notebook, compare and males have the brightest blue feathers. They are the most successful in winning mates contrast stabilizing, directional, and and have the best territories. For young buntings, the brightest blue and the dullest disruptive selection. brown males are more likely to win mates than males with bluish-brown feathers. Research suggests that dominant adult males are aggressive toward young buntings they see as threats, including bright blue and bluish-brown males. The dullest brown birds can therefore win a mate because the adult males leave Explore Online them alone. Meanwhile, the bright blue birds attract mates Disruptive Selection simply because of their color. FIGURE 12: In disruptive selection, the extreme phenotypes are selected over the intermediate phenotypes. The type of selection observed in male lazuli bunting birds is called disruptive selection. This is the type of natural selection mean in which both extreme phenotypes (brown and bright blue feathers) are favored, while individuals with the intermediate phenotype (in between brown and blue) are selected against. Frequency In disruptive selection, both extreme phenotypes are favored, while intermediate forms are selected against. The middle of the distribution graph is disrupted: individuals with genotypes that code for intermediate phenotypes are less successful that those with genotypes that code for extreme phenotypes. By favoring both extreme phenotypes, disruptive Range of variable selection can lead to the formation of new species. Explain Using evidence from this lesson, explain why populations, and not individuals, evolve. Lesson 1 Evolution of Populations 425  EXPLORATION 3 Effects of Gene Flow Predict Explain how the Roses can grow in the wild or be cultivated. A bee may transport pollen from a farm difference in gene flow that cultivates roses of different colors to a nearby area where wild red roses grow. The between populations could cause pollen can fertilize a wild rose flower, introducing new genetic material into the wild them to evolve in different or population. This is an example of gene flow, which is the movement of alleles from one similar ways. population to another. Gene flow can cause a population to evolve. Genetic Drift Small populations are more likely to be affected by chance than large populations. Let’s look at how a chance event can affect the alleles that code for a lizard’s tail shape. Hands-On Activity Modeling Population Changes Use a deck of cards to represent the lizard population. The four suits represent four different alleles for tail shape. The allele frequencies of the original population are 25% spade, 25% heart, 25% club, and 25% diamond tail shapes. Predict How can random chance affect the allele frequencies in a population? MATERIALS PROCEDURE deck of cards 1. Shuffle the cards. Holding the deck face down, turn over 40 cards. These cards represent the alleles of 20 offspring produced by random mating of the individuals in the initial population. 2. Separate the 40 cards by suit and then find the allele frequencies for the offspring by calculating the percentage of each suit. Record these values in your Evidence Notebook. 3. Suppose a storm isolates a few lizards on another island where they start a new population. Reshuffle the deck and draw 10 cards to represent the alleles of five offspring produced in this smaller isolated population. 4. Repeat Step 2 to calculate the resulting allele frequencies. Record the results in your Evidence Notebook. ANALYZE Answer the following questions in your Evidence Notebook: 1. Compare the original allele frequencies to those calculated in Steps 2 and 4. How did they change? 2. Does this activity demonstrate evolution? Why or why not? Does it demonstrate natural selection? Why or why not? 426 Unit 9 Patterns of Evolution  What you observed in the activity is called genetic drift, which is a change in allele frequencies due to chance. For example, chance events such as natural disasters or birds dropping seeds on an island can change allele frequencies in a population. This phenomenon of genetic drift is typically observed in small populations because small populations are more likely to be affected by chance alone than large populations. The chance event causes some alleles to decrease in frequency, which may cause them to eventually disappear from the population all together. It causes other alleles to increase in frequency and possibly become fixed in the population. Scientists have identified two processes that can cause population sizes to decrease enough for genetic drift to occur. Each of these processes results in a population with different allele frequencies than those that existed in the original population. Bottleneck Effect In the late 1800s, northern elephant seals were severely overhunted for their blubber, which was used in lamp oil. It is estimated that by 1890, there were fewer than 100 individuals left. After hunting ended, the population rebounded, and now there are more than 100,000 individuals. FIGURE 13: The hunting of northern elephant seals greatly depleted the species’ numbers and genetic diversity. Analyze Use the model in Figure 13 to explain the initial change in genetic variation population between the initial elephant seal population and the population after it rebounded. population in 1890s current population The northern elephant seal suffered from the bottleneck effect. This is genetic drift resulting from an event that drastically reduces the size of a population. Through genetic drift, some alleles can be completely lost from the gene pool and others can be Image Credits: (l) ©National Geographic Magazines/Marc Moritsch/Getty Images fixed in the population, resulting in lower genetic diversity. Founder Effect The Old Order Amish communities were founded in North America by small numbers of migrants from Europe. The gene pools of these smaller populations are very different from those of the larger populations. For example, the Amish of Lancaster County, Pennsylvania have a high rate of Ellis-van Creveld syndrome. Although this form of dwarfism is rare in other human populations, it has become common in this Amish population through genetic drift. Geneticists have traced this syndrome back to one of the community’s founding couples. Lesson 1 Evolution of Populations 427  Consider what would happen to a population of beetles that were nearly wiped out due to a natural disaster, such as the population shown in Figure 14. The original population had high levels of genetic diversity. After the disaster, two smaller populations of beetles survived, but there was no gene flow between the populations. The descendants of founding population A would have a different gene pool from the descendants of population B. For example, founding population A had beetles with genes coding for black exoskeletons. The descendants of these individuals also had black exoskeletons. Founding population B, however, had no individuals with genes for a black exoskeleton, so this gene was lost in population B. Gather Evidence FIGURE 14: Genetic variation decreases when a small number of individuals colonize How does the genetic new areas. variation of the new population sample of descendants compare to that of the old original population population? Use evidence to founding population A support your answer. founding population B The founder effect is genetic drift that occurs when a small number of individuals become isolated from the original population and colonize a new area. Figure 14 demonstrates genetic drift due to the founder effect in a beetle population. The founding populations each represent a distinct gene pool observed in the founding population. As a result, allele frequencies within the founding populations change from the original population reducing genetic variation. Sexual Selection Male peacocks have elaborate tails made of long, colorful feathers. These tail feathers not only make male peacocks easy targets for predators, they also make flying away from predators harder. Female peacocks, though, are a muted, brown color and do not possess long tail feathers like the males. These flashy colors and ornamental traits FIGURE 15: The winner of a fight seem to be in contrast with what should have evolved from natural selection, so how increases his chances of mating did they evolve? Image Credits: (b) ©Corbis/W. Perry Conway/Getty Images with a female. In general, mating is less costly to a male than a female. Males produce many sperm, so they can invest in mating without much cost. Females, on the other hand, produce a limited number of offspring. They tend to select males that will give their offspring the best chance of survival. This difference in reproductive costs can make females choosier than males about mates. Sexual selection occurs when certain traits increase reproductive success. Prior to the mating season, male animals like deer, elk, and moose fight other males. The winner in this competition establishes his dominance over other males and his fitness to mate with the females in the population. This type of competition among male members for the right to mate is known as intrasexual selection. 428 Unit 9 Patterns of Evolution  The superb bird of paradise, like other species of birds of FIGURE 16: The male superb bird of paradise has bright paradise, engages in courtship behavior that increases mating feathers and large plumes to attract females. success by attracting females. Superb males have feathers on their backs that are not used for flying. During courtship, the male birds use the back and chest feathers to form a funnel- like structure around their heads. This posture highlights their bright-colored breast feathers. They also flick their feathers and dance. Other birds of paradise have bright colors, large plumes, and long tail feathers and perform dances to attract the attention of females. Intersexual selection is a form of sexual selection in which males display certain traits that attract females. Males involved in intersexual selection are often more brightly colored, have larger features, or have other characteristics to attract females. In birds of paradise, long feathers, bright plumes, and courtship behavior are due to Collaborate With a intersexual selection. These traits are costly to develop, so males who possess them partner, discuss what a are usually healthy and strong. Scientific data show that, in some species, bright colors female might learn about a male indicate parasite resistance. Sick males may have muted coloring and likely do not through his color, size, and possess characteristics attractive to females. Females are able to pick the males in the ornamental features, like bright best condition or that have better genes for mating. tail feathers. Stability and Change A population is stable and in genetic equilibrium when its genetic makeup does not change over time. Because the conditions that lead to this genetic stability are rare in the natural world, evolution occurs. There are five mechanisms that can lead to evolution: Mutation can lead to the formation of new alleles. Mutations produce genetic variation. Natural selection affects populations, acting on traits that increase an individual’s ability to survive and reproduce. Sexual selection selects for traits that give members of a population a competitive advantage in mating and reproducing. Image Credits: ©National Geographic Magazines/Tim Laman/Getty Images Genetic drift affects small populations and is caused by random events that affect the population. Gene flow occurs when individuals move in and out of populations. This movement introduces and removes alleles from the gene pool. Explain Why is genetic drift more likely in small populations than in large populations? Consider the male ruffs from the beginning of this lesson. How could genetic drift or sexual selection explain the different types of males in the population? Use evidence from the lesson to support your claims. Lesson 1 Evolution of Populations 429  CONTINUE YOUR EXPLORATION Data Analysis Antibiotic-Resistant Bacteria Antibiotics are medicines used to N. gonorrhoeae Resistance, United States, 1987-2011 kill disease-causing bacteria. Studies have shown that certain species of FIGURE 17: N. gonorrhoeae shows some level of resistance to many types of antibiotics. disease-causing bacteria evolved to be 30 Tetracycline resistant to antibiotics. The Centers for resistance Disease Control and Prevention (CDC) 25 Penicillin found that doctors were prescribing resistance antibiotics when they weren’t necessary. 20 Fluoroquinolone Additionally, patients were not taking resistance Percent their full antibiotic doses. Both practices 15 Reduced have led to bacteria developing cefixime resistance against various antibiotics. 10 susceptibility Consider a population of bacteria. In the population, most bacteria have 5 genes that make them susceptible to antibiotics, but a very small percentage 0 1987 1992 1997 2002 2007 2012 of the population do not have these Year genes. These bacteria are antibiotic Source: The Gonococcal Isolate Surveillance Project (GISP), resistant. When exposed to antibiotics, quoted in "Antibiotic Resistance Threats in the United States, 2013" (CDC) the bacteria population experiences the bottleneck effect. The bacteria in N. gonorrhoeae is also developing fluoroquinolones around 2000. This the population that are susceptible to resistance to the drug cefixime. It is can be attributed to the increased use antibiotics are killed. The remaining recommended that cefixime be used of this antibiotic during this time. It resistant individuals reproduce, with other antibiotics, or not at all, also shows that the bacteria has been passing on the resistance genes to so the bacteria do not become fully resistant to penicillin since the 1980s their offspring. Eventually, the resistant to it. and continues to be resistant. For this population consists of more antibiotic- reason, scientists need to continuously resistant bacteria. Figure 17 shows resistance patterns develop new antibiotics to treat of N. gonorrhoeae. This graph The bacterium N. gonorrhoeae causes gonorrhea. However, new antibiotics shows an increased resistance to the disease gonorrhea. This disease can lead to new resistances. affects organs of the reproductive system, as well as parts of the urinary tract. If not treated, an affected Data Analysis person may lose the ability to produce offspring. The bacteria are transferred Answer the following questions in your Evidence Notebook: from one person to another through 1. What happened to penicillin resistance from 1987 to 1990? sexual activity. 2. What type of natural selection is observed in antibiotic-resistant bacteria? N. gonorrhoeae has now developed 3. Make a model to show the changes in the population of bacteria over varying levels of resistance to most time as they are were exposed to antibiotics. antibiotics, including penicillin. EXAMINING NATURAL SELECTION IN RUNAWAY Go online to choose one of SELECTION AFRICAN SWALLOWTAILS SELECTION these other paths. 430 Unit 9 Patterns of Evolution  EVALUATE Lesson Self-Check CAN YOU EXPLAIN IT? FIGURE 18: There are three types of male ruffs, and all can occur in a single population. a An independent male b A satellite male c A female. Faeder males greatly resemble females. Recall that there are three types of males in the ruff population. The dominant “independent” males are territorial and fight other independent males to attract females. The smaller “satellite” males do not fight. Satellites freely move between independents’ territories and are able to mate with some females. The “faeder” males look like female ruffs. They generally mate with females sneakily while the other males are distracted or fighting. Explain Refer to the notes in your Evidence Notebook to explain how three very different types of males evolved in a single population. Scientists think that the independent males expend a lot of energy and incur the Image Credits: (l) (r) ©Arco Images GmbH/C. Wermter/Alamy; (c) ©Matthijs Kuijpers/Alamy risk of being injured in a fight when establishing a territory to attract females. The independents (84 percent of the population) attract females by showing dominance. Types that pay fewer of these costs also have evolved within the population. The satellites (14 percent of the population) mate with the females in the independent males’ territories. Though independent ruffs may mate with more females, they are at risk of being injured in territorial fights and are more susceptible to predators because of their elaborate plumage and larger size. The faeders (1 percent of the population) are able to reproduce by sneaking into an independent male’s territory and quickly mating with a female. Interestingly, scientists have discovered that the behavior and physical traits that differentiate the three types are controlled at a single genetic location, a “supergene.” Studies indicate that the faeders are a result of a chromosome inversion that occurred 3.8 million years ago. The satellite type was a result of a chromosomal rearrangement between the original sequence and the inverted sequence that happened about 0.5 million years ago. The differences in traits and behavior among these types allow them all to be successful and persist in the population. Lesson 1 Evolution of Populations 431  EVALUATE CHECKPOINTS Check Your Understanding 7. Widowbirds are members of a bird species found in the southeastern part of Africa. The females have dull brown feathers and the males have black feathers, including tail Use the following information to answer Questions 1-4. feathers that measure an average of 41 centimeters long. Studies have shown that females prefer and choose to In a population of 900 pea plants, 530 are homozygous mate with males that have longer tails. Which outcome purple, 250 are heterozygous purple, and 120 are can be expected to occur in this scenario? homozygous white. Purple color (P) is dominant and white color (p) is recessive. a. Over time, there will be more males with 41 centimeters tails. 1. Determine the genotypic frequency in the population for b. Over time, there will be more males with tails longer PP, Pp, and pp individuals. than 41 centimeters. c. Over time, there will be more males with tails shorter 2. What is the total number of alleles in this gene pool? than 41 centimeters. d. Over time, there will be more males with no tails. 3. What is the allele frequency of P? Express the frequency as a decimal rounded to the nearest hundredth. 8. Model how the bottleneck effect can lead to evolution by putting the following events in order. 4. What is the allele frequency of p? Express the frequency a. Many of the individuals die in the population. as a decimal rounded to the nearest hundredth. b. Population increases with less variation. c. A random event acts on a population. Use the information in the table below to answer Question 5. d. Surviving individuals reproduce. Frequency in Original Frequency in New Color Variation 9. Determine if the scenarios will likely result in an increase Population (%) Population (%) or a decrease in genetic variation over time. Copy and Gray 15 45 then complete the table below in your notebook by Gray and white 60 20 writing “increase” or “decrease” in the second column. White 25 35 Genetic Variation within Scenarios Individual Population 5. The frequencies of a color trait among rabbits living in a Mosquitos become resistant to mountainous area have changed over time. What type of pesticides. selection most likely occurred? Arabian horses mate with wild a. directional horses. b. disruptive c. stabilizing A population becomes lactose d. sexual intolerant through mutation. A smaller body is selected for in 6. Scientists observed a population of monkeys on an island. cheetahs. The monkeys were observed to have different finger lengths. Some monkeys had long fingers, some had short fingers, but the majority of them had finger lengths that were closer to the short finger length. Explain how this trait in the population of monkeys would evolve over time if tree branches on the island grew thicker. Would this be an example of stabilizing, directional, or disruptive selection? 432 Unit 9 Patterns of Evolution  MAKE YOUR OWN STUDY GUIDE 10. The Florida panther is a type of mountain lion. About a hundred years ago, Florida panthers scattered and In your Evidence Notebook, design a study guide that mated with other subspecies of mountain lions in nearby supports the main ideas from this lesson: populations. Changing allele frequencies can be an indication of the a. Explain how the gene flow in this population would be evolution of a population. affected by the introduction of the Florida panthers. b. Would genetic variation increase or decrease in the Selective pressures, such as competition and predation, can mountain lion population? shift the distribution of traits in a population. Small populations are more susceptible to genetic drift 11. Give an example of the way sexual selection can cause because large populations are able to lessen the impact of extreme phenotypes in a population. random events. Remember to include the following information in your study Use the information in the table below to answer Question 12. guide: Trait Frequency of Trait (%) Use examples that model main ideas. Record explanations for the phenomena you investigated. Predicted Frequencies Observed Frequencies Use evidence to support your explanations. Your support can Color Variation Using the Hardy- after Three include drawings, data, graphs, laboratory conclusions, and Weinberg Equation Generations other evidence recorded throughout the lesson. Large flowers 75 44 Consider how the evolution of populations relates to the Medium flowers 10 22 assumption that natural laws operate today as they did in the past Small flowers 15 34 and will continue to do so in the future. 12. Study the table to compare predicted and actual frequencies of flower size in a flower population. What conclusion is best supported by the data in the table? a. The population is evolving. b. The population’s gene pool remained the same. c. The population is in equilibrium. d. The population selected for an intermediate trait. 13. Why must allele frequencies in a gene pool always add up to 100 percent? 14. Explain how the process of genetic drift occurs completely by chance. 15. What are the differences and similarities between natural selection and sexual selection? Lesson 1 Evolution of Populations 433