Reading comp PrepTest 101 · Section 4 · Question 17

Topic: Science

Paragraph 1

• Paragraph note
  • "Latitudinal gradient" phenomenon (more species near the equator than poles)
• Views, minor Meta-Structures, and the author's attitude
  • Comparison:
    • When you compare a habitat type that stretches out from the north to the south, there are more species near the equator than far from the equator (first sentence)
  • Author's attitude: "probably even more pronounced" (last sentence)

Paragraph 2

• Paragraph note
  • "Time theory" hypothesis and author's rebuttal
• Views, minor Meta-Structures, and the author's attitude
  • Cause-and-effect relationship, according to the "time theory" hypothesis:
    • Latitudinal gradient exists because tropics haven't been interrupted by ice ages and therefore have more time to develop species (first sentence)
  • Comparison, according to the "time theory" hypothesis:
    • Tropic zones have not been affected by ice ages, while temperate and arctic zones have (first sentence)
  • Author's view:
    • The "time theory" hypothesis isn't consistent with the evidence, as arctic conditions haven't actually been interrupted by ice ages, and there is a varying range of interruption in temperate areas (last sentence)
  • Author's attitude: "However" (last sentence)

Paragraph 3

• Paragraph note
  • Species-energy hypothesis and author's rebuttal
• Views, minor Meta-Structures, and the author's attitude
  • Cause-and-effect relationship, according to the species-energy hypothesis:
    • Latitudinal gradient exists because tropics have more energy from the sun, which causes high biomass and lower rates of extinction (first and last sentences)
  • Author's view:
    • The causal mechanism between biomass and increased local populations remains untested, and biomass and the number of species are not necessarily correlated (second through last sentences)
  • Author's attitude: "However" (second sentence); "remains untested" (last sentence)

Paragraph 4

• Paragraph note
  • Climatic-stability hypothesis and author's rebuttal
• Views, minor Meta-Structures, and the author's attitude
  • Cause-and-effect relationship, according to the climatic-stability hypothesis:
    • The tropics' stable climate causes a reliable resource supply, which allows more species to coexist (first through third sentences)
  • Author's view
    • The climatic stability hypothesis describes a local process that cannot explain the regional differences along the latitudinal gradient (fourth and last sentences)
  • Example of a local process, according to the author:
    • Competition (last sentence)
  • Example of regional differences along the species gradient, according to the author:
    • Forests along the equator and forces at higher altitudes (last sentence)
  • Author's attitude: "However" (fourth sentence); "cannot account for" (fourth sentence)

Paragraph 5

• Paragraph note
  • Rate-of-speciation hypothesis and author's endorsement
• Views, minor Meta-Structures, and the author's attitude
  • Cause-and-effect relationship, according to the rate-of-speciation hypothesis:
    • Latitudinal gradient exists because, in the tropics, the rate of speciation (the development of new species) is higher, and the extinction rate of species is lower (last sentence)
  • Author's attitude: "most plausible hypothesis" (first sentence)

Paragraph 6

• Paragraph note
  • How rate of speciation works
• Views, minor Meta-Structures, and the author's attitude
  • Cause-and-effect relationship/comparisons, according to the rerate-of-speciation hypothesis:
    • Speciation occurs when subgroups become isolated and differentiate from the main population (first and second sentences)
    • Subgroups are more likely to survive in the tropics than in the arctic, causing a higher rate of speciation and a lower rate of extinction in the tropics (third and last sentences)

Main Point: The rate-of-speciation hypothesis is the best explanation for why there exists a latitudinal gradient in species distribution.

Paragraph 1, Sentence 1 (P1, S1) - Description of phenomenon

P5, S1 - The author's preferred explanation

P6, S4 - How the preferred explanation actually works 

Phenomenon/Explanation: This passage follows a Phenomenon/Explanation Meta-Structure. Passages that utilize this Meta-Structure follow a consistent pattern. First, the passage will describe an observable fact, behavior, or situation. Then, the author will present one or more explanations of that observable fact, behavior, or situation. That's exactly the model this passage follows. The author describes our phenomenon — the latitudinal gradient of species distribution — and then presents four hypotheses that could explain this phenomenon.

In this type of Meta-Structure, if the author says that one explanation is the best, then that's the main point of the passage. Here, the author prefers the last hypothesis, "rate of speciation." So the main point is that the rate-of-speciation hypothesis is the best explanation for why there exists a latitudinal gradient in species distribution. 

Lists: This most prominent minor Meta-Structure in this passage is a list. In fact, nearly the entire passage is dedicated to listing four separate hypotheses for the latitudinal gradient of species distribution. When the whole passage is organized around a list, we'll surely get at least a few questions about items on this list. This is why our notes must allow us to quickly determine which paragraph describes which hypothesis.

Causality: Like many science passages, cause and effect also figure heavily into this passage. Each hypothesis of the latitudinal gradient of species distribution proposes a causal mechanism to explain the phenomenon. For those of us who are less fluent in science than we'd like to be, it can help if we simplify the cause-and-effect relationships as much as possible. We've done so below for each hypothesis:

• "Time theory": The lack of ice-age disturbance in the tropics is the cause, and more time for species to emerge in the tropics is the effect (P2, S1)
• Species-energy: Greater energy from the sun is the cause, and more biomass, lower rates of extinction, and more species are the effects (P3, S2)
• Climatic stability: The tropics' stable climate is the cause, and more resources and species are the effects (P4, S1-3)
• Rate of speciation: The better chance of surviving in the tropics is the cause, and the lower rates of extinction and higher rates of survival in the tropics are the effects (P6, S4)

The nice thing about this passage is that the middle paragraphs are formulaic. The second, third, and fourth paragraphs follow the same formula: the author describes a hypothesis and then dismisses the hypothesis with counterevidence. Even when the topics (like ecology and species distribution) are unfamiliar, noticing these patterns can help us follow the passage and decoct the structure.