Chapter 1. Chapter 14: Natural Selection and Adaptation

1.1 Introduction

Interactive Study Guide
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Polaris Trail

Welcome to the Interactive Study Guide for Chapter 14: Natural Selection and Adaptation! This Study Guide will help you master your understanding of the chapter's Driving Questions, using interactive Infographics and activities, as well as targeted assessment questions. Click "Next" to get started, or select a Driving Question from the drop-down menu to the right.

Bugs that Resist Drugs:

Drug-resistant bacteria are on the rise. Can we stop them?

DRIVING QUESTIONS

  • What is staph and can it be present in the absence of an infection?
  • How do bacteria resist the effects of antibiotics?
  • How do populations evolve and what is the role of evolution in antibiotic resistance?

1.2 Driving Question 1

Driving Question 1

What is staphylococcus and can it be present in the absence of an infection?

Why should you care?

Staphylococcus aureus (commonly called staph) is a common skin bacterium that is easily transmitted from person to person. A strain called Methicillin-Resistant Staphylococcus aureus (MRSA) is becoming increasingly common. This strain is potentially deadly and very difficult to treat.

What should you know?

To fully answer this Driving Question, you should be able to:

  1. Describe the basic features of the bacterium Staphylococcus aureus (S. aureus) and its modes of transmission.
  2. Explain why people can harbor S. aureus and not be infected by it.

Infographic Focus

The infographic most pertinent to the Driving Question is 14.1.

Describe the basic features of the bacterium Staphylococcus aureus (S. aureus) and its modes of transmission.

Question 1.1

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S. aureus is spherical and grows in clusters. (Under a high-powered microscope, S. aureus looks like a bunch of grapes.) It is primarily found on the skin and in the nose and throat of about one-third of the U.S. population.

Question 1.2

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Since it is primarily found on the skin, S. aureus can cause pimples, boils, and wound infections. In immunocompromised populations (like the elderly, newborn, and ill), S. aureus is more likely to cause serious, potentially life-threatening disease by infecting the bloodstream or lungs.

Question 1.3

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S. aureus is transmitted from person to person through skin-to-skin contact. A person can also become colonized with S. aureus by handling contaminated objects or touching contaminated surfaces.

Explain why people can harbor S. aureus and not be infected by it.

Question 1.4

d1iWe0jLK1E9otWgH+J+NzLe1OB0+eajyopOKhR8xw8epcPl9b6nCONL1b+DHViwbTzptpTzaCnwJy6Kw/4s7WsenTuan7EsrvOZV4Xf9nG8FiVN0XsNflV8GSRq9GuEWfOb1lst9d7CYpLpZu+HAJAKa6LkmDe1Nu7iHqkkmkNAp5VOJArWMh/rIfDZBXMvCjgl2NLnnyAui6rVlfedHB8T6XYWk+U5/5Y+tszS7aKqNf43qF6ySL2hcLN5N9MBFTZ0TzKWhyQ=
People with healthy immune systems are typically able to fend off an infection by S. aureus. Your skin is your body’s first defense against pathogen attack. As long as it is in tact, pathogens cannot easily gain access to your bloodstream, where they can do serious damage. It is when the immunocompromised, or people with open wounds from cuts or scrapes, come in contact with S. aureus that there may be a problem.

Review Questions

Question 1.5

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Question 1.6

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Question 1.7

b1rfR+fxqJn3Drlxw7xmk1WXqhh6VQaFTX0RAvlI3H5jFyl6Uh0iAj9XJnTB9XQQrUIqNwQqR6eQzp72jZK/73ahSmoJb8Q0tbtYJtXvkny8pQL7xounHh72gOv5iD5YPb5Wv4w+3FXVPBCtenB/uwWICgk8mzReRqdogXKpyWFMNAtxKw3GGYId1fJyGa+Zgq0jkriPM/74ND0ra5ARvP+/RocgXcoLxcj8yv6C2tNJaL4UzVyT1MMRVacIcLOk64ef3nHSxjJ+9FJmUvEzhpOKfYRnwMfpZl+i4+pZDOHFIdmlprEzkY2c61FJ6N915PS33uCzmsi/nKK+BflpOwFhPJq+ipCxgZpm5N2YNZ6zoUd4gt9LUUykJrabXgwcDTukv287zhnhLO6WMTAPkFAxoZykHnBrrGKgh4jrKiTPzHGZa9U+M0PCxj1TEcmQ3WYT3JXIR1mDazuCkZFwtA==
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1.3 Driving Question 2

Driving Question 2

How do bacteria resist the effects of antibiotics?

Why should you care?

Beta-lactams are among the most commonly prescribed antibiotics; you have probably taken them several times yourself. MRSA is resistant to this class of antibiotics.

Ricky Lannetti, Rebecca Lohsen, and Carlos Don died because a bacterium that is normally harmless became resistant to antibiotics. Genetic variability is at the heart of the process that led to that change. Bacteria reproduce asexually through a process called binary fission, so their primary source of genetic variability is mutations that occur during DNA replication. Because bacteria reproduce extremely rapidly, bacterial populations can accumulate mutations rapidly. The more mutations that occur, the greater the chances that one or more of them will be beneficial to the bacteria – and potentially deadly to us. Bacteria can also acquire new genes via direct transfer from other bacteria. As a consequence, once it arises via mutation, a gene for resistance to one or more antibiotics can spread very rapidly through a bacterial population (including a population inside your own body!). This process is key to the problem of widespread antibiotic resistance.

What should you know?

To fully answer this Driving Question, you should be able to:

  1. Diagram and describe how beta-lactam antibiotics work.
  2. Outline the steps in bacterial reproduction.
  3. Discuss the consequences of binary fission for the accumulation of mutations in a bacterial population over time.
  4. Describe the process of gene swapping/gene transfer.
  5. Illustrate and describe how binary fission, mutation, and gene swapping/gene transfer interact to create diverse bacterial populations, some of which may acquire resistance to an antibiotic.

Infographic Focus

The infographics most pertinent to the Driving Question are 14.2, 14.3 and 14.4.

Question Test Your Vocabulary

Choose the correct term for each of the following definitions:

Term Definition
GTivRj3JxMDr53SWhRfhvldw56nvAECa4dzr/EvfYRM= A type of asexual reproduction in which one parental cell divides into two.
l5fgIBqTAZ9cJtcjBEhyXNF8zDGYrU+JfZ/I5eBX4xQ= Chemicals that either kill bacteria or slow their growth by interfering with the function of essential bacterial cell structures.
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Describe how beta-lactam antibiotics work.

Question 1.8

iF3Ra/u3Me7go1rHDBAfddMIt6bmp+5Wpiw44ROhAl7wlzRn6O09Und+UGQlVIwXPMp/SA==
It is a ridged structure that prevents the high pressure of the water inside the cell from rupturing it.

Question 1.9

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Beta-lactam antibiotics interfere with the proper synthesis of the bacterial cell wall. Since the antibiotic makes the cell wall faulty and weak, the high pressure of the water inside the cell can no longer be contained, and the water bursts out and ruptures the cell, killing the bacterium.

Outline the steps in bacterial reproduction.

Question 1.10

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  1. A single bacterium elongates and replicates its DNA.
  2. Next, the cell wall and plasma membrane begin to divide.
  3. The cell wall and plasma membrane start to form around the divided DNA, effectively bisecting the original bacterium.
  4. The two daughter cells separate, each with its own cell wall, plasma membrane, and DNA.

Discuss the consequences of binary fission for the accumulation of mutations in a bacterial population over time.

Question 1.11

Assume for simplicity’s sake that you are growing a colony of bacteria from a single cell. One mutation occurs during the first cell division and is maintained in the colony during each round of replication. (Multiple bacteria may be dividing and mutating, but only one bacterium is maintained in the colony.) Replication occurs every 20 minutes.

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Since binary fission produces two daughter cells that have genomes identical to that of the parent, a mutation in the second generation would therefore affect half of the total population. Bacterial growth by binary fission is exponential (2n, where n = the number of rounds of replication). At the end of 1 hour, there will have been three rounds of replication, resulting in eight bacteria (23 = 8) . Half of them, four, will have the mutation. At the end of one day, there will have been 72 rounds of replication, resulting in 4.7 × 1021 bacteria (272 = 4.7 × 1021). Half of them, 2.35 × 1021, will have the maintained mutation.

Describe gene transfer.

Question 1.12

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Another way bacteria acquire new genetic information is by transferring genes to each other. Genes are transferred from one bacterium to another through a tubelike structure called a pilus. Genes that are transferred are usually on a small circular chromosome called a plasmid. This method does not require binary fission, as the bacteria do not have to divide to accomplish the transfer.

Illustrate and describe how binary fission, mutation, and gene swapping, or gene transfer, interact to create diverse bacterial populations, some of which are resistant to an antibiotic.

Question 1.13

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Review Questions

Question 1.14

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Question 1.15

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1.4 Driving Question 3

Driving Question 3

How do populations evolve, and what is the role of evolution in antibiotic resistance?

Why should you care?

The traits that led to MRSA arose via mutation and gene swapping. The bacterial populations that possess those traits have increased in number, however, because we have created an environment that favors them. The interaction between an organism’s phenotype and its environment determines its fitness – its ability to survive and reproduce relative to other organisms in the same environment. The ways in which we use antibiotics (and antivirals, insecticides, herbicides, etc., for that matter) create the conditions that increase the fitness of organisms resistant to those substances.

Natural selection explains why MRSA has spread. Although several different mechanisms can lead to evolutionary change, natural selection is the only mechanism that leads to adaptation – the accumulation of traits favorable in a particular environment. Understanding adaptation is important for scientists in fields as varied as agriculture, medicine, resource management, and ecology as well as in evolutionary biology. On a more immediate level, understanding the process will help you make better health care decisions!

When we understand the patterns of natural selection, we improve our ability (very limited at present) to predict how a given environmental change – whether it’s the introduction of a new antibiotic or the current pattern of global climate change – might affect us and the plants, animals, and microbes around us. The better we can predict these changes, the better we can manage our health and resources.

MRSA and other forms of antibiotic resistant bacteria pose serious health risks. Fortunately, a few simple steps can reduce your risks.

What should you know?

To fully answer this Driving Question, you should be able to:

  1. Illustrate and explain the relationship among an organism’s phenotype, environment, and fitness.
  2. Identify and explain the characteristics of individuals, populations, and the environment that are necessary for natural selection to occur.
  3. Outline the specific steps of natural selection.
  4. Describe the relationship between natural selection and evolution.
  5. Compare and contrast directional, stabilizing, and diversifying selection.
  6. Give at least one example of each pattern of natural selection.
  7. List and describe four practices that will help reduce your risk of becoming infected by antibiotic-resistant bacteria.

Infographic Focus

The infographics most pertinent to the Driving Question are 14.4 14.5, 14.6, 14.7 and 14.8.

Question Test Your Vocabulary

Choose the correct term for each of the following definitions:

Term Definition
la9AnAt7K/b2mwYLZHCtNideGjC59s58NEoess7W36nz+lpgoF65zlGtUJ9Hk7YpT7RjogMCgnIqBByz6MixFLXI7yveF0+cCsaXWv4eLwmWhnQAHLbEF+pOnTwYbuw/0n+W2X1w2Fo+da6VWpuQ3VVYyuEeCVXfePd+8mclZ5N6FdFS The relative ability of an organism to survive and reproduce in a particular environment.
ZRIQt5wamf3LgsfdhUZxdPHBWIw8t3mX06MxiTDetp01kDeMWdXkCsr2Cy1bqxj5AkUHoWESgWWDajxGg0QdYlDTdepCgLhb2AQ9XFCE09a4d1ddWfnyjF/WUbq5XXl5DOdS8NO2EHqQfe6K9r2M6Mj8Jqdg70IGLWT+4NAEC/y1Qonj A type of natural selection in which organisms with phenotypes at one end of a spectrum are favored by the environment.
+vf3Gtdc5hMdyAEz7Ch41dRu5oMsD3tSaKPVP9LMBlR3KGOvVOftoiCg586/OJGWJ5JjJsDcSoDIQ6QIt580bBd+XlzKMIezHFywCtN2DKWE5bWKXduYfRz118wZiD9s5uCpSopwE/tLcP2uo5swDZoI76w9zGHsrcgcbYJwG+CVVlQ2 Differential survival and reproduction of individuals in response to environmental pressure that leads to change in allele frequencies in a population over time.
A23dVSMar3J6xBRrCl+TCV5KgqtV+3d291m3zfokMxZWfJM/9EU3cH16Yexb+NuEjGB8U0ou7MEL0YOPK2anHTNGt97ErekUXiaF1xRKl2zhlF8J5y6dRTAYHxFC7kETwRhaRyjPY+cwWH7t87q0uocvH1M9fz21xsdWDVJ9063Ghq5p A type of natural selection in which organisms with phenotypes at both extremes of the phenotypic range are favored by the environment.
yKZuG2fFlqCnbBVoobbvvT6q0CnIn/tygzT5ef/+vsQY71C1hEIWdcDHYhaoBT/5f1XEBKuOzAC6NkHpdoxz9Ii5EppB4DzG/A7Jcf15GtQ9X+Y9+oYNEMb5XXIVkYGZ/ZdsdSN7Jq2TRbU5Zmv5NLOulUNNUl8osKzMSV5h4adieKqc Change in allele frequencies in a population over time.
qktMgvfypo4M5Fbk8W50rX1peGkwZKtpIIT5GNPB94o/yNNM/Cy1d0xRgXvlhXhZEibl94VEEggrao9BUJgwoxE0CS4sJAAgaEHVXEJrN19E0c7I8EupnVc5aXoIoLvbSLLlSdE5SU0Oll05LJ4ByZk+b57by7NoFY+i8GU8hcH6msye A type of natural selection in which organisms near the middle of the phenotypic range of variation are favored by the environment.
DtFWJkqF/2ji6n4wf5tSHY12bnd4ctwleOtRckjj+rcrIJHkcV0IrRnCgrsgrTlbZSmjb0nz9bUqWkgN36JSykSrwDE0IzSpqRuPu9uq9y0CACSXd1KygJc3mZ83mEZQVlxgu1CQRdNbYtzX9Wlylh2jwrD3pBprzyGpHEAfq4pfZy0Q A group of organisms of the same species living together in the same geographic area.
6frvyjCob/dYi6G8ZYSoE6i0Zgo4zltsQ66Hk+zP9XTnIgeXAGpy94StTzoK0aYyZX8F4mrfeAD/+IPuJWKwCBNQrxld1KDSiiGEcNQBT7seYZbjyy8/CD2fMw6qRZqnBc8i/YdJE3ad04Zr5dLAXkksBwuOUkI2aV/BQsPlpKff2RA4 The process by which populations become better suited to their environment as a result of natural selection.
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Illustrate and explain the relationship among an organism’s phenotype, environment, and fitness.

Question 1.16

Fill in the table with the appropriate fitness levels to illustrate the interaction between a population’s phenotype and its environment on its fitness:

Bacterial Phenotype Fitness in the absence of antibiotic Fitness in the presence of antibiotic
Antibiotic-sensitive LU/yM02nlYRE6AmYkP5/HeY78u0=
A single mutation in a tumor suppressor gene associated with DNA repair.
LU/yM02nlYRE6AmYkP5/HeY78u0=
The cell is bigger and there is double the amount of organelles.
Moderately antibiotic-resistant LU/yM02nlYRE6AmYkP5/HeY78u0=
DNA replicates resulting in chromosome duplication.
LU/yM02nlYRE6AmYkP5/HeY78u0=
There is twice as much DNA in the cell.
Highly antibiotic-resistant LU/yM02nlYRE6AmYkP5/HeY78u0=
DNA replicates resulting in chromosome duplication.
LU/yM02nlYRE6AmYkP5/HeY78u0=
Correct.
Table

2
Bacterial Phenotype Fitness in the absence of antibiotic Fitness in the presence of antibiotic
Antibiotic-sensitive Same as rest of population Low
Moderately antibiotic-resistant Same as rest of population Medium
Highly antibiotic-resistant Same as rest of population Medium
Table
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Question 1.17

Suppose that in a population of flies some individuals carry a mutation making them resistant to the insecticide DDT but reducing the number of offspring they can produce. Flies without the mutation have many offspring but die in the presence of DDT. Draw a table similar to the previous one, substituting “DDT” for “antibiotic” and “fly” for “bacteria.” Fill in the table with the appropriate fitness levels.

Fly Phenotype Fitness in the absence of DDT Fitness in the presence of DDT
DDT-sensitive LU/yM02nlYRE6AmYkP5/HeY78u0=
A single mutation in a tumor suppressor gene associated with DNA repair.
LU/yM02nlYRE6AmYkP5/HeY78u0=
The cell is bigger and there is double the amount of organelles.
Moderately DDT-resistant LU/yM02nlYRE6AmYkP5/HeY78u0=
DNA replicates resulting in chromosome duplication.
LU/yM02nlYRE6AmYkP5/HeY78u0=
There is twice as much DNA in the cell.
Highly DDT-resistant LU/yM02nlYRE6AmYkP5/HeY78u0=
DNA replicates resulting in chromosome duplication.
LU/yM02nlYRE6AmYkP5/HeY78u0=
Correct.
Table

2
Fly Phenotype Fitness in the absence of DDT Fitness in the presence of DDT
DDT-sensitive Same as rest of population Low
Moderately DDT-resistant Medium Medium
Highly DDT-resistant Low High
Table
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Question 1.18

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An organism’s fitness is the relative ability of that organism (and hence its phenotype) to survive and reproduce in its environment.

Identify and explain the characteristics of individuals, populations, and the environment that are necessary for natural selection to occur.

Question 1.19

Polaris Trail

Examine the top figure in Infographic 14.6 and answer these questions:

WCj97f+QAY21tvLnjbXZLFXhnDf3JQsbdCd5rsgOMxH+7AxETc7IcV8UOnGimUPZ1OAFG5vTKmJkDHclu5A0pJcwfdMPZIIm+UoBMYxVfBOOd4X7
Genetically diverse

Question 1.20

Z2/aUMKicKAKktjUQmKtUx7UNR6Owkkze+nDilWoDoIspTvABRqmWKPUqFOI90ai
Absence of antibiotic

Question 1.21

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No

Question 1.22

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No, it remains the same

Question 1.23

Polaris Trail

Examine the bottom figure in Infographic 14.6 and answer these questions:

WCj97f+QAY21tvLnjbXZLFXhnDf3JQsbdCd5rsgOMxH+7AxETc7IcV8UOnGimUPZ1OAFG5vTKmJkDHclu5A0pJcwfdMPZIIm+UoBMYxVfBOOd4X7
genetically diverse

Question 1.24

Z2/aUMKicKAKktjUQmKtUx7UNR6Owkkze+nDilWoDoIspTvABRqmWKPUqFOI90ai
Presence of antibiotic

Question 1.25

TuwIDnnZpGmX8g1LqBlewi2GM3xUEQMY5h5Aymb776wKsmMnYmiUNu9jCL8lo2nH6v4W+wmZ6tLVXyr6s71b0dLEvDLU5A00DwJSwXJyqrnP3n/NNawKqfxBR5hlZYxMO5Rcr9Dke/eiYUZZ
Yes, the ones that carry a gene making them resistant to the antibiotic will have more fitness.

Question 1.26

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Yes, the frequency of resistant alleles and phenotypes in this population will increase.

Question 1.27

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Natural selection cannot occur in a population with no genetic variation. Pressure on the population from the environment will not lead to a change in allele frequencies because there were no differences in alleles to begin with. Natural selection does not necessarily occur anytime there is a population with genetic variation. For natural selection to occur, there has to be pressure from the environment to change the allele frequencies. If there is no pressure, there is no natural selection, even in the presence of genetic variation.

Question 1.28

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No, there would be no change in allelic frequencies of the population before and after the environmental pressure event.

Question 1.29

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For natural selection to occur, there has to be genetic variation in the population. The pressure from the environment will determine which traits are favored.

Outline the specific steps of natural selection.

Question 1.30

Polaris Trail

A population of birds consists of individuals exhibiting a range of bill sizes. Under normal conditions, most individuals find sufficient food in the form of seeds to survive and reproduce at about the same rate. During prolonged droughts, however, smaller seeds are consumed rapidly, leaving larger, harder seeds as the primary food source. Birds with larger bills are better able to crack and eat these seeds than are birds with smaller bills.

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Under normal conditions, the food supply would remain stable with both large and small seeds to eat. Thus, the frequency of small-beaked birds to large-beaked birds would remain stable as well (there would be no major change).

Under drought conditions, small seeds would be eaten rapidly leaving mainly large seeds for consumption. Since the small-beaked birds cannot eat the large seeds, they would die off in the population resulting in a lowered frequency of small-beaked birds to large-beaked birds

Describe the relationship between natural selection and evolution.

Question 1.31

A small population of endangered beetles lives in a habitat so isolated that no other members of the species can immigrate. Most individuals in the population are brown, black, and green, and a few individuals are brown, black, and blue. Not all beetles can find enough food to reproduce every year. One year, by chance, none of the brown, black, and blue individuals are able to reproduce, leaving a population of only black, brown, and green individuals.

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Yes, there was a change in the allelic frequency over time.

Question 1.32

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No, there was no pressure that changed the allelic frequencies, since it was by chance that the brown, black, and blue beetles were not able to reproduce.

Question 1.33

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The evolution of a population of organisms is the result of a change in allelic frequencies over time. When that change is due to an environmental selection pressure, it is called natural selection.

Compare and contrast directional, stabilizing, and diversifying selection.

Question 1.34

Polaris Trail

Examine Infographic 14.7.

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All three patterns show a shift in allelic frequency, presumably due to an environmental pressure.

Question 1.35

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They differ in which alleles, along the spectrum of the allele population, are selected for.

Give at least one example of each pattern of natural selection.

Question 1.36

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• Directional selection: resistant strains of bacteria in an antibiotic environment
• Stabilizing selection: human birth weight
• Diversifying selection: beak size of the African finch

Question 1.37

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• Directional selection: a desert floods, selecting for the flipper rabbit, which has webbed feet.
• Stabilizing selection: a bird population's migratory path leads them through a tunnel of a certain diameter filled with spider webs. Only the birds that are small enough to fit through the tunnel yet large enough to break free of the spider webs will survive.
• Diversifying selection: a population of stiff noodle neck turtles has three main phenotypes. Those with long necks eat the berries off tall trees. Those with medium necks eat the berries off shrubs. Those with short necks eat the berries off the ground. Since the stiff noodle neck turtle cannot bend its neck, it relies on the berries found where it can reach them. If a ground fire affecting only medium to short foliage spreads through the forest, the surviving phenotypes of turtles would be tall neck and short neck.

List and describe four mechanisms for reducing your risk of becoming infected by antibiotic-resistant bacteria.

Question 1.38

Polaris Trail
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• Disinfect common surfaces; this reduces the transmission of antibiotic-resistant bacteria.
• Wash hands with regular soap; this reduces spread of antibiotic-resistant bacteria and self-infection.
• Get vaccinated; this prevents you from becoming ill with certain strains of antibiotic-resistant bacteria.
• Do not take needless antibiotics; if you have a viral infection, do not take antibiotics, which will needlessly expose your commensal bacteria to the drug and thus introduce selection pressure to the populations.


Thought Question: Why is the increasing bacterial resistance to antibiotics a bad thing?

Review Questions

Question 1.39

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Question 1.40

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Question 1.41

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Question 1.42

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2
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