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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 18. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What has caused the disappearance of over 200 Lake Victoria cichlid species over the last 30 years?
  2. How is the practice of dog breeding similar to the process of adaptive evolution?
  3. What unique adaptations of the “four-eyed fish” have enabled this species to survive in its environments?
  4. In terms of speciation, how does reproductive isolation differ from geographic isolation?
  5. What is a ring species?
  6. Which animal species have been known to develop as a result of polyploidy?
  7. What role does adaptation play in our fight against HIV infection?

A Guide to the Reading

When exploring the content in Chapter 18 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.

Reproductive Isolation

As defined in the chapter, a species is a group of organisms that are capable of interbreeding with each other.  Different species remain as separate groups because they are not able to reproduce with members of other species.  This can be either because of geographical isolation or reproductive isolation.  Individuals from different species are incapable of interbreeding, therefore they do not exchange genetic information and remain genotypically and phenotypically distinct.  Species are said to be reproductively isolated from other species as a result of these reproductive barriers.  Reproductive barriers can take several forms, including those which prevent successful mating (due to ecological, behavioral, mechanical, or gametic isolation) and those which prevent the survival or the success of hybrid species (zygote death, hybrid performance) when mating between two species occurs.

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

  • In Section 18.3, Species are reproductively isolated from one another
  • Table 18.1, Barriers That Can Reproductively Isolate Two Species in the Same Geographic Region

Allopatric vs. Sympatric Speciation

Speciation occurs when a single species evolves into two or more distinct species over a period of time.  The two main mechanisms by which speciation occurs are allopatric speciation and sympatric speciation.  The key to understanding the difference between these mechanisms is to realize that allopatric speciation occurs when new species are formed as the result of the geographic isolation of populations of the original species.  Over time, these separated populations may evolve independently and become distinct in terms of both genotype and phenotype.  It is important to note that the distance separating the two populations must be great enough to inhibit gene flow between the populations.  This factor makes the distance needed to achieve geographic separation vary depending upon the species (i.e. organisms capable of flight will be able to easily cross new rivers or canyons).  Sympatric speciation is the most common mechanism by which species form.  It is important to note that sympatric speciation does not involve the geographic isolation of groups.  Instead, other factors, such as polyploidy (multiple sets of chromosomes) or the natural selection of alleles that favor the use of one food source over another, may result in the formation of distinct species within the same geographic region.

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

  • In Section 18.4, Speciation often results from geographic isolation
  • Section 18.4, Speciation can occur without geographic isolation
  • Figure 18.9, Allopatric Speciation

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:

Meiosis and Chromosome Number (Ploidy)

  • Chapter 9 – Section 9.4, Meiosis: Halving the Chromosome Number

Genetic Control of Development

  • Chapter 14 – in Section 14.3, Development in eukaryotes relies on gene cascades and master-switch genes


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

Gene Flow

  • Chapter 17 – Section 17.5, Gene Flow: Exchanging Alleles between Populations

Natural Selection

  • Chapter 17 – Section 17.7, Natural Selection: The Effects of Advantageous Alleles

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