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Chapter 18: Problem 56

The human immunodeficiency virus (HIV) reproduces very quickly. A single virus can replicate itself a billion times in one 24-hour period. In a hypothetical treatment situation, a patient鈥檚 HIV population consists entirely of drug- resistant viruses after just a few weeks of treatment. How can this treatment result best be explained? How does this explanation illustrate that evolution is an ongoing process? a. The resistant viruses passed their genes to the non-resistant viruses so that 100% of the viruses became resistant. This illustrates evolution as an ongoing process because the genes of the population changed in real time. b. The non-resistant viruses died, and the resistant ones survived and rapidly reproduced. This illustrates evolution as an ongoing process because the change in the HIV population is the result of natural selection. c. The viruses developed resistance to the drug after repeated exposure to it. This illustrates evolution as an ongoing process because the viruses were able to adapt to changing conditions. d. The drug-resistant viruses were more fit than their non-resistant counterparts to begin with, and over time they dominated the population. This illustrates evolution as an ongoing process because natural selection favored one phenotype over another.

Short Answer

Expert verified
Option D is correct. Natural selection favored drug-resistant HIV viruses.

Step by step solution

01

Understand the Problem

A patient's HIV population becomes entirely drug-resistant after a few weeks of treatment. The task is to identify which explanation best illustrates how this evolution occurred.
02

Evaluate Each Option

Consider each given option to see how well it explains the change in the HIV population and its connection to evolutionary principles.
03

Option A Analysis

Option A suggests that resistant viruses pass their genes to non-resistant viruses, leading to 100% resistance. This is incorrect since viruses cannot pass genes this way.
04

Option B Analysis

Option B suggests non-resistant viruses died and resistant viruses survived and reproduced. This aligns with natural selection, where the environment (the drug) selects for drug-resistant viruses.
05

Option C Analysis

Option C implies the viruses developed resistance after exposure. This is less correct because it underemphasizes the pre-existence of some resistant viruses and overstates the adaptive mutation within a short period.
06

Option D Analysis

Option D states that drug-resistant viruses were already more fit and dominated the population over time, which is accurate. This explanation ties directly to natural selection favoring drug-resistant phenotypes.
07

Final Selection

Considering the analysis, the correct explanation is Option D, which best describes the process of natural selection and illustrates ongoing evolution.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Natural Selection
Natural selection is a fundamental concept in biology that explains how populations evolve over time. It revolves around the idea that individuals within a population vary in their traits, and these variations can impact their survival and reproduction. In the context of HIV, the virus population includes both drug-resistant and non-resistant strains. When a patient undergoes treatment, the drug acts as a selective pressure, killing off the non-resistant viruses while allowing the resistant ones to survive.

Because the drug-resistant viruses have a survival advantage, they reproduce more rapidly. In a short time, the population becomes dominated by drug-resistant strains. This process highlights how natural selection works, favoring traits that increase an organism's fitness in a given environment. The key takeaways are:
  • Variation exists within a population.
  • Some traits provide a survival or reproductive advantage.
  • Organisms with advantageous traits are more likely to survive and reproduce.
  • Over time, these traits become more common in the population.
Evolution
Evolution is the change in the genetic composition of a population over generations. It's an ongoing process driven by mechanisms such as natural selection, mutation, genetic drift, and gene flow. In the scenario with the HIV population, evolution is evident in how the drug-resistant viruses dominate over time.

Initially, the patient鈥檚 HIV population might have a mixture of drug-resistant and non-resistant viruses. After exposure to the drug, non-resistant viruses are less likely to survive, and resistant viruses thrive. The genetic makeup of the virus population shifts towards resistance. This change over time in the frequency of traits within the population is evolution in action.

The steps in this HIV scenario illustrate key aspects of evolution:
  • Genetic Variation: There are different strains of the virus with varying resistance.
  • Selection Pressure: The treatment creates an environment where only resistant viruses survive.
  • Reproduction of the Fittest: Resistant viruses continue to replicate, passing on their traits.
  • Change in Population Over Time: The population becomes predominantly drug-resistant.
This process shows evolution happening in real time, underlining its dynamic nature.
Virus Replication
Virus replication is the process by which viruses reproduce and create new virus particles. HIV, like other viruses, relies on a host cell to replicate. The replication process is rapid 鈥� a single HIV virus can produce over a billion copies in just 24 hours. This high replication rate accelerates evolutionary processes like natural selection and the development of drug resistance.

Here鈥檚 a simplified look at HIV replication:
  • The HIV virus attaches to a host cell, usually a type of white blood cell called a T-helper cell.
  • HIV injects its genetic material into the host cell.
  • The host cell machinery is hijacked to produce viral components.
  • These components are assembled into new virus particles.
  • The new viruses burst out of the host cell, ready to infect other cells.
Because HIV replicates so quickly, any mutation that provides an advantage, such as drug resistance, can rapidly spread through the population. This rapid replication and mutation rate are why HIV can adapt so quickly to treatment pressures, leading to drug resistance as a significant challenge in managing HIV infections.

Understanding the virus replication process is crucial in comprehending how quickly resistance can develop and spread. Moreover, it underscores the importance of developing treatment strategies that can outpace viral evolution.

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Most popular questions from this chapter

What pattern in the fossil record would you expect to see to support the model of gradual speciation? How would you expect this pattern to differ from a pattern in the fossil record that supports the model of punctuated equilibrium? Explain. a. In the case of gradual speciation, the fossil record would show only a few hybrid individuals, followed by individuals of the two distinct species. For the case of punctuated equilibrium, the fossil record would show many hybrid individuals persisting through several geological layers. b. In the case of gradual speciation, the fossil record would show the parent species in a single location, such that the newly diverged species remained in contact with each other. For the case of punctuated equilibrium, the fossil record would show a geographic divide within the parent species that caused it to diverge into multiple new species. c. In the case of gradual speciation, the fossil record would show many intermediate forms. For the case of punctuated equilibrium, the fossil record would show new forms that persist essentially unchanged through several geological layers, then disappear just as a new form appears. d. Gradual speciation would be undetectable in the fossil record. For the case of punctuated equilibrium, the fossil record would show a steady progression of distinct forms.

Which component of speciation would be least likely to be a part of punctuated equilibrium? a. a division in populations b. a change in environmental conditions c. ongoing gene flow d. a number of mutations occuring at once

Prior to 1800 in England, the typical moth of the species Biston betularia (peppered moth) had a light pattern. Dark colored moths were rare. By the late 19th century, the light-colored moths were rare, and the moths with dark patterns were abundant. The cause of this change was hypothesized to be selective predation by birds (J.W. Tutt, 1896). During the industrial revolution, soot and other wastes from industrial processes killed tree lichens and darkened tree trunks. Thus, prior to the pollution of the industrial revolution, dark moths stood out on light-colored trees and were vulnerable to predators. With the rise of pollution, however, the coloring of moths vulnerable to predators changed to light. Commonly used in biology textbooks, the peppered moth is a classic example of evolutionary change in action. The example describes changes in a population鈥檚 allele frequencies-a small-scale change, evolutionarily speaking. The presence of both light and dark forms within the gene pool is demonstrated by the story, but the peppered moth stays a peppered moth. Which scenario, if it were to occur, would be a model for large-scale evolutionary change? a. Conditions change such that the dark form of the moth is favored and the light form is diminished in the population due to predation. Conditions change again, the dark form is vulnerable, and the light form returns to prevalence. b. Conditions change such that the dark form of the moth is favored and the light form is eradicated in the population due to predation. Conditions change again, the dark form is vulnerable, and the dark form is eradicated due to predation. c. Conditions change such that dark form of the moth is favored and the light form is diminished in the population due to predation. Conditions change again, and both forms have equal prevalence. d. Conditions change such that dark form of the moth is favored and the light form is eradicated in the population due to predation. Conditions change again, the dark form is vulnerable. It develops an adaptation that shields it from predation.

Describe a situation where hybrid reproduction would cause two species to continue divergence. a. f two closely related species continue to produce hybrids, the hybrids will compete with both species, causing them to find new niches which will further their divergence b. If two closely related species continue to produce hybrids, they will develop reproductive barriers to prevent production of hybrids, to ensure that they remain separate species. c. If two closely related species continue to produce hybrids that are less fit than the parent species, there would be reinforcement of divergence. d. f two closely related species continue to produce hybrids they will always converge into a single species.

Which are two primary sources of genetic variation? a. mutations and sexual reproduction b. isolation and sexual reproduction c. sexual reproduction and asexual reproduction d. migration and sexual reproduction
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