Reading Comprehension

PT101 · S4 · P3 · Q18 Species Gradient

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When the same habitat types (forests, oceans, grasslands, etc.) in regions of different latitudes are compared, it becomes apparent that the overall number of species increases from pole to equator.

Topic

The increase in species diversity from poles to equator: why there are more species in the tropics than at higher latitudes.

Framework

Answer / Explain

Main Point

The passage explores several hypotheses to explain why there are more species near the equator than near the poles but favors the idea that higher speciation (and not higher extinction) in the tropics best explains this pattern. (Most valuable sentence: Last sentence of P5: )

P1: The Pattern—Latitudinal Diversity Gradient

Scientists have noticed that as you move from the poles toward the equator, the number of species in similar habitats increases, with the effect likely being even bigger than currently known.

P2: The Time Theory Hypothesis

One idea ("time theory") says tropical regions have more species simply because they've had more time without disruptive ice ages for new species to evolve, but this doesn't fully account for differences among various regions.

P3: The Species-Energy Hypothesis

Another hypothesis suggests that more sunlight in the tropics means higher growth and reproduction rates, which could lead to more biomass and thus more species, but the supposed chain of cause-and-effect has several weak links and is unproven.

18.

Which one of the following, if true, most clearly weakens the rate-of-speciation hypothesis as it is described in the passage?

  1. A remote subgroup of a

    Strengthens, if anything

    This rate of speciation hypothesis involves the idea of subgroups getting separated from the original population, but surviving long enough to form a new species (i.e. which means they can't interbreed with the original group). So this answer describes something that aligns with the hypothesis.

    4% picked this

  2. Investigation of a small area

    No Impact / Strengthens, if anything

    This doesn't really tell us anything that hurts any part of the rate of speciation's story. If anything it helps support the idea that when a subgroup gets isolated in a tropical environment, it has a better chance of surviving and becoming a new species (since it's possible for many competing species to coexist in the same small area).

    7% picked this

  3. A correlation between higher energy

    No Impact

    This answer sounds more like it's trying to strengthen the species-energy hypothesis than weakening the rate of speciation hypothesis. It doesn't have anything to do with any of the causal chain described by the rate of speciation hypothesis. We could argue whether this even strengthens the species-energy hypothesis (it affirms correlations, but the author's skepticism was whether you could actually test for the causal mechanism that more energy causes bigger populations .... this answer is just saying the correlation is definitively established). But either way, this has nothing to do with hurting the rate of speciation hypothesis, whereas the correct answer directly attacks a part of that hypothesis.

    6% picked this

  4. Researchers find more undiscovered species

    No Impact

    Finding more species near the poles would somewhat diminish the hi/low contrast of the species gradient (maybe the arctic region doesn't have WAY fewer species than tropical latitudes; it just has moderately fewer species). But this answer has nothing to do with the rate of speciation hypothesis. This answer is trying to weaken the idea that there is even a species gradient in the first place.

    11% picked this

  5. Correct

    Most of the isolated subgroups

    Why this is right

    This answer directly attacks the link from 2 to 3, in this causal chain. more uneven more subgroups more spread of a >> isolated >> survive >> species species over subgroups formed large area The rate of speciation hypothesis maintained that in the Tropics, unlike in higher latitudes, an isolated subgroup would be more likely to survive and turn into a new species. This answer strongly undercuts that notion by saying that most isolated subgroups in a tropical zone experienced rapid extinction.

    Skill tested: Weaken · how this choice captures the passage's function is the move to repeat next time.

    72% picked this

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