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Ch. 22 - Descent with Modification: A Darwininan View of Life
Chapter 22, Problem 3

Within six months of effectively using methicillin to treat S. aureus infections in a community, all new S. aureus infections were caused by MRSA. How can this best be explained? a. A patient must have become infected with MRSA from another community. b. In response to the drug, S. aureus began making drug- resistant versions of the protein targeted by the drug. c. Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency. d. S. aureus evolved to resist vaccines.

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1
Understand the problem: The question is asking for the best explanation for why all new S. aureus infections were caused by MRSA (Methicillin-resistant Staphylococcus aureus) within six months of using methicillin.
Analyze the options: Option (a) suggests external introduction, (b) suggests a change in protein production in response to the drug, (c) suggests pre-existing resistance and selection, and (d) incorrectly refers to vaccine resistance, not drug resistance.
Recall the principles of antibiotic resistance: Antibiotic resistance can occur through mutation and natural selection. If resistant bacteria are present, they can survive the antibiotic treatment and reproduce, leading to a higher proportion of resistant bacteria.
Apply the concept of natural selection: In the context of antibiotic use, natural selection would favor the survival and reproduction of bacteria that are resistant to the antibiotic. Over time, these resistant bacteria become more common.
Choose the best explanation: Option (c) 'Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency.' This option correctly applies the principles of natural selection and the presence of pre-existing resistant bacteria.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Natural Selection

Natural selection is a fundamental mechanism of evolution where organisms better adapted to their environment tend to survive and produce more offspring. In the context of antibiotic resistance, when antibiotics are used, susceptible bacteria are killed off, while those with mutations that confer resistance survive and reproduce, leading to a higher frequency of resistant strains in the population.
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Natural Selection

Antibiotic Resistance

Antibiotic resistance occurs when bacteria evolve mechanisms to resist the effects of drugs that once killed them or inhibited their growth. This can happen through genetic mutations or acquiring resistance genes from other bacteria, often exacerbated by the overuse or misuse of antibiotics, leading to the emergence of strains like MRSA (Methicillin-Resistant Staphylococcus aureus).
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Evolution of Antibiotic Resistance in Bacteria

Mutation and Adaptation

Mutations are changes in the DNA sequence of an organism that can lead to new traits. In bacteria, these mutations can result in drug resistance, allowing them to survive in the presence of antibiotics. Over time, beneficial mutations can spread through a population, leading to adaptations that enhance survival in challenging environments, such as those created by antibiotic treatment.
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Related Practice
Textbook Question

Which of the following is an observation or inference on which natural selection is based? (A) Individuals do not vary in their heritable characteristics. (B) Only well-adapted individuals produce offspring. (C) Species produce more offspring than the environment can support. (D) Nearly all of each individual's offspring will survive and reproduce.

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Textbook Question

Which of the following observations helped Darwin shape his concept of descent with modification? a. Species diversity declines farther from the equator. b. Fewer species live on islands than on the nearest continents. c. Birds live on islands located farther from the mainland than the birds' maximum nonstop flight distance. d. South American temperate plants are more similar to the tropical plants of South America than to the temperate plants of Europe.

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Textbook Question

DNA sequences in many human genes are very similar to the sequences of corresponding genes in chimpanzees. The most likely explanation for this result is that a. humans and chimpanzees share a relatively recent common ancestor. b. humans evolved from chimpanzees. c. chimpanzees evolved from humans. d. convergent evolution led to the DNA similarities.

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Textbook Question

The upper forelimbs of humans and bats have fairly similar skeletal structures, whereas the corresponding bones in whales have very different shapes and proportions. However, genetic data suggest that all three kinds of organisms diverged from a common ancestor at about the same time. Which of the following is the most likely explanation for these data? a. Forelimb evolution was adaptive in people and bats, but not in whales. b. Natural selection in an aquatic environment resulted in significant changes to whale forelimb anatomy. c. Genes mutate faster in whales than in humans or bats. d. Whales are not properly classified as mammals.

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Textbook Question

A swim bladder is a gas-filled sac that helps fish maintain buoyancy. The evolution of the swim bladder from the air-breathing organ (a simple lung) of an ancestral fish is an example of a. exaptation. b. changes in Hox gene expression. c. paedomorphosis. d. adaptive radiation.

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