12.4 Patterns of Selection

Transcription

1 12.4 Patterns of Selection DID YOU KNOW? Selection of Another Hemoglobin Allele? In November 2001, geneticists reported in the journal, Nature, about another mutated form of the hemoglobin gene that provides resistance to malaria. Unlike the sickle-cell allele (HbS), individuals with two copies of the HbC allele show no signs of disease and are very resistant to even the most serious form of malaria. Interestingly, the mutation in the -hemoglobin that produced the HbC allele is in the identical location as the HbS allele. What is the source of variation? How are subtle differences passed from generation to generation? These questions that puzzled Darwin have been answered by the scientific understanding of genetics and mutations. Mutations provide a continuous supply of new genetic variations, which may be inherited and expressed as different phenotypes. Natural selection leads to a variety of outcomes when this genetic variation occurs within competitive populations living under diverse environmental conditions. Sickle-cell anemia, a potentially serious blood disorder, is a useful example of how mutation, genetic variation, and the environment result in different patterns of natural selection. The allele for sickle-cell anemia differs from the normal hemoglobin gene by having a single base-pair mutation. Individuals homozygous for the sickle-cell allele are severely afflicted with this disorder. Heterozygous individuals are only mildly affected by sicklecell anemia; however, they are much more resistant to malaria than are people with normal hemoglobin. In regions where malaria is uncommon, individuals with the sicklecell allele are at a disadvantage and their phenotypes are less likely to contribute alleles to the gene pool. But in regions where malaria is common (Figure 1), heterozygous individuals are strongly favoured; they are much more likely to survive and pass on their genes to the next generation. The environment selects the best-adapted phenotype and, in so doing, favours a particular set of alleles. The sickle-cell allele is only common where it provides an overall advantage to the individual. In populations where it has an overall harmful effect, it does not persist. This pattern establishes an important relationship between mutations and evolution: Harmful mutations occur frequently but they are selected against and, therefore, these mutant alleles remain extremely rare. Beneficial mutations are rare but they are selected for and, therefore, these mutant alleles accumulate over time. Although genes provide the source of variation, natural selection acts on individuals and their phenotypes. As a result, particular alleles are most successful and passed on when they enhance the phenotype of the individual and, thereby, contribute to their reproductive success. Selective forces can favour particular variations in the phenotype of individuals in a number of ways. (a) (b) Figure 1 Of the 120 million new cases of malaria each year, about 1 million are fatal. The prevalence of malaria in Africa (a) closely matches the distribution of the sickle-cell allele (b). P. falciparum malaria in Africa malaria sickle-cell allele in Africa 1 5% 5 10% 10 20% Adapted from Biology 5/E, fig p. 407 Peter H. Raven and George B. Johnson, the McGraw-Hill Companies, Inc. 556 Chapter 12 NEL

2 Section 12.4 Types of Selection Many factors influence how selection can operate on individual phenotypes in a population. Some populations live for long periods of time under stable conditions, while others live in a constantly changing environment. At times, a variable or diverse environment may favour multiple phenotypes. Within populations themselves, competition for mates adds further selective pressures that can and do influence the evolutionary pathway followed by populations. These different sets of conditions result in a number of general selection types. Stabilizing Selection In your lifetime, you are unlikely to see a dramatic change in the appearance of any species as a result of natural selection. Indeed, most species show little change over periods lasting thousands of years. Although these observations suggest the absence of evolutionary processes, theoretical models predict that once species become well adapted to their environment, selection pressures will tend to prevent them from changing. Stabilizing selection occurs when the most common phenotypes within a population are most favoured by the environment. For example, human birth weights are variable and part of this variability is inheritable. According to the theory of natural selection, babies born at weights that offer the best chance of surviving birth should be more numerous, and research shows that far more human babies are born weighing just over 3 kg than any other weight. Babies with significantly lower weights are often developmentally premature and less likely to survive, while heavier babies often experience birth-related complications that threaten the life of both baby and mother. The evidence suggests that natural selection eliminates extreme variations of a particular trait, resulting in a population in which most human births are near the ideal weight. Stabilizing selection is by far the most common form of selection. Once a species becomes adapted to its environment, selective pressures maintain their evolved features and traits. For example, the hummingbird draws nectar from flowers with a long bill and tongue (Figure 2). Most birds of this species will succeed if their bill and tongue length are well adapted for the size of flowers that they feed on in their local environment. A longer bill requires more nutrients and energy to grow and carry around, while a shorter bill may reduce a bird s ability to reach food. The ideal bill length also increases the success of flower pollination, thereby enhancing the reproductive success of the plant species as well as that of the hummingbird. The environment will select against mutations that occur to produce birds with a bill length that differs from the best-adapted length. The graphs in Figure 3 show the effects of stabilizing selection based on bill length over successive generations of hummingbirds. DID YOU KNOW? Bigger Babies In societies with a high standard of nutrition and medical care, birth weights may be changing. Healthy mothers are producing bigger babies, while hospitals providing Caesarean sections and incubators reduce many risks to heavier and smaller babies. Selective pressures are no longer as pronounced. What long-term effects might result from these conditions? stabilizing selection selection against individuals exhibiting variations in a trait that deviate from the current population average Figure 2 (a) Range of Values for Bill Length, (b) Range of Values for Bill Length, (c) Time 1 Time 2 Range of values for Bill Length, Time 3 Figure 3 Stabilizing selection in a population of hummingbirds. The bell-shaped curve shows the frequency and variation in hummingbird bill lengths. The arrows indicate the less successful forms. NEL Mechanisms of Evolution 557

3 Figure 4 Directional Selection Directional selection occurs when the environment favours individuals with more extreme variations of a trait. When an organism migrates to a new environment, or when aspects of its habitat change, it will encounter new forces of natural selection. If the hummingbird population moves to a new habitat with longer flowers, individuals with bills that were best adapted to medium-length flowers will no longer be ideal (Figure 4). Birds that inherited longer bills will then be more successful than those with medium-length and short bills; longer-billed birds will obtain more food and contribute more offspring to later generations (Figure 5). Directional selection may result in an observable change in a population. (a) Range of Values for Bill Length, (b) Range of Values for Bill Length, (c) Time 1 Time 2 Range of Values for Bill Length, Time 3 Figure 5 Directional selection shown in a population of hummingbirds. In a new environment with longer flowers, which hummingbirds have an advantage? directional selection selection that favours an increase or decrease in the value of a trait from the current population average In species with large populations and short generation times such as salmon many offspring are produced. In such species, the amount of genetic variation from both recombination and mutation is increased. Gill-net fishing in the Bella Coola River and the Upper Johnston Strait from 1950 to 1974 had a dramatic effect on the pink salmon population: the average weight of the salmon decreased by about one third (Figure 6) Bella Coola odd years Figure 6 Gill-net fishing substantially decreased the size of adult salmon on a British Columbia river over 25 years. Larger fish were less likely to escape gill nets and, therefore, less likely to contribute to later generations. Distinctly different salmon populations breed in alternating years in each location. At the time of this study imperial measurements were in use. Mean Weight (lb) even years Upper Johnston Strait odd years even years Year 558 Chapter 12 NEL

4 Section 12.4 In these years, about 75% of all adult salmon were being caught and, as a result, smaller adults were the most likely to escape the nets and contribute offspring to the next generation. In this way, human activity resulted in directional selection. Disruptive Selection Disruptive selection favours individuals with variations at opposite extremes of a trait over individuals with intermediate variations. Sometimes, environmental conditions may favour more than one phenotype. For example, two species of plants with different-sized flowers may be available as a food source for the hummingbird population (Figure 7). Each species is a good source of nectar, but neither is well suited to a hummingbird with an average-length bill. Birds with longer and shorter bills will be more successful and will contribute more offspring to later generations (Figure 8). Figure 7 (a) Range of Values for Bill Length, (b) Range of Values for Bill Length, (c) Time 1 Time 2 Figure 8 Disruptive selection in a hummingbird population. In a habitat with both shorter and longer flowers, which birds have the advantage? A living example of disruptive selection in a bird population is provided by the blackbellied seedcracker finch (Pyrenestes ostrinus). These African finches depend on the seeds of two different types of sedge, one that produces a soft seed and the other a much harder seed. Finches with small bills, shown in Figure 9(a) are efficient at feeding on soft seeds, while birds with larger bills, shown in Figure 9(b) are able to crack the hard seeds. A study of 2700 finches produced the findings depicted in the graph shown in Figure 9, which appears to be observable evidence for disruptive selection. Disruptive selection is a significant evolutionary mechanism for the formation of distinctive forms within a population. Distinctive groups may eventually become isolated breeding populations with separate gene pools. Range of Values for Bill Length, Time 3 disruptive selection selection that favours two or more variations or forms of a trait that differ from the current population average 60 (a) narrow bill NEL Width of Lower Bill (mm) (b) wide bill From Biology: The Unity and Diversity of Life w/cd ROM, 9/E, by C. Starr and Taggart Reprinted with permission of Wadsworth, an imprint of the Wadsworth Group, a division of Thomson Learning. Fax Figure 9 The environment has strongly favoured the evolution of two different bill sizes among black-bellied seedcracker finches. They display variation in bill sizes, but during the dry season, survival is limited to two narrow size ranges (orange bars on graph). This is an observable product of disruptive natural selection. Mechanisms of Evolution 559

5 sexual selection differential reproductive success that results from variation in the ability to obtain mates; results in sexual dimorphism and mating and courtship behaviours sexual dimorphism striking differences in the physical appearance of males and females not usually applied to behavioural differences between sexes Sexual Selection Individuals that mate and reproduce frequently make a substantial contribution to the gene pool of later generations. Sexual selection favours the selection of any trait that influences the mating success of the individual. The traits favoured in sexual selection include sexual dimorphism (i.e., striking differences in the physical appearance of males and females) and behavioural differences between the sexes. The most common forms of sexual selection result from female mate choice and from male-versus-male competition. In some species, females choose mates based on physical traits, such as bright colouration, or behavioural traits, such as courtship displays and song. In other species, males are equipped with physical features that assist them in establishing control of and defending their territory against other males (Figure 10). This territory provides an area to which they can attract, and sometimes forcibly detain, the females with which they mate. Such traits are not produced by selective pressures from environmental conditions; if they were, both sexes would be expected to possess them. Figure 10 Sexual dimorphism may take the form of a physical advantage, such as a much larger size in males, or an enlarged limb, as in fiddler crabs. DID YOU KNOW? Penguin Puzzlement Not all species show obvious sexual dimorphism. In some species of penguin, males and females look so similar that even they have a hard time telling each other apart (Figure 11). A male picks up a stone and drops it at the feet of a would-be mate. If the other penguin happens to be a male, the gift is firmly rejected. Figure 11 Many species have evolved features that are a compromise between different selective pressures. Sexual selection has produced traits that are beneficial for mating but are otherwise detrimental. Avoiding predators is not made easier, for instance, by brilliant plumage or a distinctive song (Figure 12). A bizarre extreme sometimes called runaway selection can be illustrated by the stalk-eyed fly (Figure 13). Although males and females both have eye stalks, females have preferentially mated with males with the longest eye stalks to the point where the feature has become very exaggerated. Sexual diversity is not limited to animal populations. Most plants do not select mates but they do need to attract or use various agents such as insects, birds, and bats to assist in pollination. Flowers and scents are the most obvious sexual features that have evolved through natural selection. By maximizing their chances of being pollinated, plants have a greater likelihood of contributing more alleles to the next generation s gene pool. Chance and selection play important roles in evolution. Scientists accept that anatomical, morphological, and behavioural traits of organisms evolve chiefly through selection mechanisms. These traits directly influence the daily survival and reproductive success of the individual; at the molecular level, the roles of random chance and selection are less clear. Some biologists argue that most of the differences in genomes result from neutral mutations, while others hold that most differences are responses to subtle effects from selective pressures. Although the debate over the details of molecular diversity continues, the basis for evolution is agreed: the ultimate fate of all genetic combinations rests in their ability to produce individuals best adapted to survive and reproduce in their habitat. Natural selection is the mechanism that drives the evolution of species. 560 Chapter 12 NEL

6 Section 12.4 DID YOU KNOW? Something Smells Fishy A study suggests that female sticklebacks detect a scent to select males with the most diverse array of genes for fighting disease. Apparently, females can use their sense of smell to find the mate that will give them the most disease-resistant offspring. Figure 12 The mating game is risky for male túngara frogs. When calling for a mate in the dark, they run the risk of giving away their location to the deadly frog-eating bat. Figure 13 This remarkable male fly continues to contribute alleles for eyes on excessively long stalks because they attract females of the species. What might change this situation? Case Study The Evolution of Antibiotic Resistance in Bacteria An article in the Canadian Medical Association Journal (July 2001) reported an alarming six-fold increase in the rate of antibiotic resistance in Canada between 1995 and In addition to added health risks, fighting antibiotic resistance can be expensive. Sunnybrook and Women s College hospitals in Toronto reported spending $ in two years on fighting resistant bacteria and, across Canada, the cost is estimated at $50 to $60 million a year. In 1996, doctors took samples of bacteria from a patient suffering from tuberculosis, a lung infection caused by the bacterium Mycobacterium tuberculosis. Cultures of the bacteria found it to be sensitive to a variety of antibiotics, including rifampin. The patient was treated with rifampin and initially responded so well that the lung infection seemed to be over. Soon after, however, the patient had a relapse and died. An autopsy revealed that bacteria had invaded the lungs again in large numbers. Cultures of these bacteria were found to be sensitive to many antibiotics, but resistant to rifampin. DNA sequencing revealed that a certain bacteria s gene had a single base-pair mutation that was known to confer resistance to rifampin. Doctors compared the new bacteria culture with the original culture and found that the sequences were identical except for this single mutation. Researchers then examined more than 100 strains of bacteria from other tuberculosis patients living in the same city at the same time. None of these bacteria had the same genetic code as the rifampin-resistant bacteria obtained in the autopsy. When doctors had begun administering rifampin, the bacteria in the patient had been subjected to a new environmental selective agent, one that gave the mutant strain a major adaptive advantage. The pattern in this story is not uncommon, but evolution offers some hope as well as alarm. Many traits that provide antibiotic resistance are harmful to the bacteria. For example, a strain of E. coli bacteria possesses a plasmid with a gene that enables it to produce an enzyme called -lactamase. This enzyme gives the bacterium resistance to the NEL Mechanisms of Evolution 561

7 DID YOU KNOW? Linezolid Linezolid, sold under the trademark name Zyvoxam, was first approved for use in Canada in May It was the first entirely new antibiotic on the market in 35 years. In the United States, where the drug had been in use for only a few months, a strain of Linezolid-resistant bacteria had already developed. antibiotic ampicillin. However, there is a cost for this resistance: to maintain its antibiotic resistance, the bacterium must devote cellular resources to producing the enzyme and to making copies of the plasmid before cell division. In another example, the bacterium Mycobacterium tuberculosis normally produces catalase, a beneficial but nonessential enzyme. This enzyme, however, activates the antibiotic isoniazid, which destroys the bacterium. Bacteria that have a defective catalase gene are, therefore, resistant to isoniazid as it cannot be activated in the absence of catalase but they lack the benefits normally provided by the enzyme. As a result of these costs of resistance, when an antibiotic is not present, natural selection often favours those bacteria that do not carry antibiotic-resistant alleles. Case Study Questions Understanding Concepts 1. Did the rifampin-resistant bacteria found in the autopsy evolve within the patient s lungs or did they result from a brand new infection? Explain the evidence. 2. Most antibiotics are derived from microorganisms that do not occur naturally in the human body. Most infectious bacteria showed no resistance to these antibiotics when they were first used in the 1940s, because pathogens (disease-causing organisms) did not already have antibiotic resistance to them then. Why? 3. Bacteria that are sensitive to antibiotics usually out-compete resistant strains in the absence of antibiotics. Account for this observation. Applying Inquiry Skills 4. Tuberculosis patients are now routinely given two different antibiotics at the same time. Why might this approach be more effective than administering a different antibiotic only after bacteria develop resistance to the first? multidrug-resistant tuberculosis Mycobacterium tuberculosis that is resistant to both antibiotics usually used to treat it: isoniazid and rifampin Making Connections 5. Suggest some strategies that could help reduce the incidence of antibiotic resistance in your own home, your school, and in society at large. EXPLORE an issue Debate: Treatment of Drug-Resistant Tuberculosis In October 1999, the Harvard Medical School reported that a deadly strain of tuberculosis (TB) was spiralling out of control. This was sixyears after TB had been declared a global health crisis. TB has been around for a long time; evidence of the bacterium that causes the lung infection Mycobacterium tuberculosis has been found in 4000-year-old Egyptian mummies. It used to have a 50% mortality rate. Presently, it is unlikely to develop in people who have a healthy immune system capable of keeping the bacterium in check, but TB is common among people with poor nutrition, a weakened immune system, and inconsistent access to shelter and medical care. As much as one third of the global human population is thought to be infected by TB. Until the late 1990s, modern drug treatments usually with the use of the antibiotics isoniazid and Decision-Making Skills Define the Issue Analyze the Issue Research Defend the Position Identify Alternatives Evaluate rifampin had produced a 98% cure rate. The 1999 report was of a worldwide outbreak of a strain of TB that was resistant to this antibiotic treatment (Figure 14). Modern medical treatment of TB requires antibiotic use from a few months to more than a year. Because these long exposures to antibiotic microorganisms increase the opportunity for TB bacteria to develop resistance, doctors have been giving patients at least two antibiotics from the outset. For people most at risk from TB, this lengthy treatment can be difficult to maintain and a challenge for medical officials to monitor, especially in regions with few clinics and trained staff. In their efforts to prevent the spread of the lethal multidrug-resistant tuberculosis, some authorities in North America are resorting to legally enforced quarantine in patients homes. 562 Chapter 12 NEL

8 Section 12.4 EXPLORE an issue continued Statement Individuals being treated for multidrug-resistant TB should be placed under medical house arrest to ensure they complete their entire treatment under medical supervision. In a group, share responsibility for researching this issue, using written and electronic resources. GO You may want to consider the current status of the TB outbreak. Some TB strains are resistant to four or more of the most effective antibiotics, which increases the cost for treating such patients at a time when medical expenses are escalating. Find out how house-arrest programs in North America operate and how effective they have been in the past. Can individual rights be balanced against society s need to prevent the spread of antibiotic-resistant diseases? Consider other alternatives to enforced confinement that could guarantee public safety. Prepare arguments and counterarguments to debate your position on this issue. Figure 14 The purple areas on the map indicate countries reporting cases of multidrug-resistant tuberculosis as of 1999 SUMMARY Patterns of Selection Neutral mutations the most common type in eukaryotic organisms usually occur in noncoding regions of genetic material but can provide additional genetic material. Harmful mutations occur frequently but the environment selects against them and, therefore, alleles that result from them remain rare. Beneficial mutations are rare but the environment selects them and, therefore, alleles resulting from them accumulate over time. The frequency of mutations in a large population is substantial. Stabilizing selection tends to maintain allele frequencies in stable environments over long periods of time. Directional selection occurs when an extreme of an inheritable trait is favoured, usually when a population experiences a change in habitat or environmental conditions. Disruptive selection favours two or more distinct forms, often as a result of a change in habitat or environmental conditions. Sexual selection favours inherited traits that enhance mating success but may reduce an individual s chances of survival. NEL Mechanisms of Evolution 563

9 Section 12.4 Questions Understanding Concepts 1. Identify and explain the type of selection that accounts for each of the following: (a) the hollow bones of birds (b) the light emitted by fireflies (c) the smell of a skunk (d) the body diameter of snakes 2. Suggest how large antlers or bright colouration could be a disadvantage for males of some species. 3. Identify and explain whether stabilizing, directional, or disruptive selection is the most likely cause of each of the following: (a) the evolution of bill size in black-bellied seedcrackers (b) the consistent colour of blue jays for the past 100 years (c) the evolution of the elephant s trunk (d) the evolution of human brain size 4. Study Figure 15. Describe the sexual dimorphism shown and its evolutionary role. Figure 15 Male (with inflated throat sac) and female frigate birds 5. Respond to this statement: Harmful mutations are more common than beneficial ones, and as the many harmful effects of mutations accumulate, species become weaker and less able to adapt to changes in their environment. Applying Inquiry Skills 6. Consider the example of natural selection and evolution in the African seedcracking finches. What specific technologies do you think might have been used by the researchers to gather, analyze, and interpret appropriate data? Be as detailed as possible. Making Connections 7. Many insect species have evolved resistance very rapidly to a range of pesticides. Like other species, insects exhibit variation in physical, chemical, and behavioural traits. (a) Describe how an insect species would evolve resistance to a pesticide newly introduced into its environment. (b) Identify and describe the type of selection the insects exhibit. (c) How might high rates of reproduction and the short duration of insect generations affect their evolution? (d) How might an understanding of the evolution of pesticide resistance influence how you use pesticides or alternative methods of insect control? 8. Many Africans who are carriers of the allele for sickle-cell anemia have emigrated from malaria-stricken areas in Africa to North America. Has this influenced the biological role of the sickle-cell allele? Explain. 9. Although, in theory, an individual could mate at random with other members of a large population, this seldom occurs. Under most natural conditions, individuals tend to mate with nearby members of the same species, especially if they are not very mobile. Alternatively, individuals choose mates that share similar traits; for example, toads (and often humans) tend to pair according to size. (a) How might inbreeding (the mating of closely related individuals) lead to an increase in recessive phenotypes? Relate your answer to either a population of cheetahs in the wild or a population of golden retrievers in a breeding kennel. (b) Does nonrandom mating result in changes to population phenotype frequencies, genotype frequencies, or allele frequencies? 564 Chapter 12 NEL