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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 17. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What caused the U.S. Surgeon General to declare that it was time to “close the book on infectious disease” in 1960?
  2. How many species of insects are estimated to be resistant to at least one pesticide?
  3. What causes human immunodeficiency virus (HIV) to act like a “moving target” in an infected individual’s body?
  4. What do the Florida panther, northern elephant seal, and the African cheetah all have in common?
  5. What legislation has caused the directional selection of light-colored moths over dark-colored moths in England over the past several decades?
  6. What is the ideal birth weight for human babies as determined by stabilizing selection?

A Guide to the Reading

When exploring the content in Chapter 17 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Genetic Drift

Genetic drift is the process by which alleles are selected within a population at random over time.  The frequency of particular alleles may change as a result of random events.  This may have a profound effect on the population if the size of the population happens to be small.  Since small populations have fewer total alleles, if a number of individuals happen to leave or are eliminated from the population, the relative effect on the remaining alleles can be drastic.  When one allele reaches a frequency of 100 percent as a result of the genetic drift of other alleles, the remaining allele is said to have reached “fixation”, that is, because this is the only allele present in the population, it will not change over time.  It is important to note that the process of genetic drift is not subject to adaptive evolution; it may result in the fixation of alleles that are beneficial, neutral, or even harmful to the population.

For more information on this concept, be sure to focus on:

  • In Section 17.6, Genetic drift affects small populations
  • Figure 17.3, Genetic Drift

Types of Natural Selection

As described in the chapter, there are three different types of natural selection, each serving to shape the evolution of a population as a result of differences in survival and reproductive success of members possessing particular traits.  Directional selection occurs when individuals possessing an extreme trait have a reproductive or survival advantage over individuals lacking this trait.  An example of directional selection might include the neck length of giraffes.  Giraffes with longer necks might have a survival advantage over animals with shorter necks since they may be more adept at feeding from tall trees.  The characteristic determining neck length would then be inherited by subsequent generations, shifting the characteristic of the population to possess longer necks.  Stabilizing selection occurs when individuals with an intermediate trait have a survival or reproductive advantage over individuals with extreme versions of the trait.  The example provided in the chapter focuses on human birth weight; underweight and/or premature babies have a survival disadvantage while very large babies are likely to cause problems during delivery.  These factors have stabilized the average birth weight of babies at around 8 pounds.  Disruptive selection occurs when individuals possessing a trait at either extreme have a survival or reproductive advantage over individuals with intermediate forms of the trait.  The example provided in the chapter focuses on the beak size within a population of African seed cracker birds.  The advantage of having either a large or small beak is that it enhances feeding efficiency, allowing populations to feed on either hard or soft seeds.  Another way to look at this is to realize that individuals possessing intermediate-sized beaks would be at a disadvantage since they would be inefficient at feeding on both types of seeds.

For more information on this concept, be sure to focus on:

  • In Section 17.7, There are three types of natural selection
  • Figure 17.6, The Three Types of Natural Selection
  • Figure 17.7, Directional Selection in the Peppered Moth
  • Figure 17.8, Stabilizing Selection for Human Birth Weight
  • Figure 17.9, Disruptive Selection for Beak Size

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Genetic Recombination

  • Chapter 11 – Section 11.3, Linkage and Crossing-Over


  • Chapter 12 – Section 12.4, Repairing Replication Errors and Damaged DNA


  • Chapter 16 – Section 16.1, Biological Evolution: The Sum of Genetic Changes

Genetic Drift and Natural Selection

  • Chapter 16 – Section 16.2, Mechanisms of Evolution


  • Chapter 18 – Section 18.1, Adaptation: Adjusting to Environmental Challenges


  • Chapter 18 – Section 18.4 , Speciation: Generating Biodiversity

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