Self-study
P1
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. ████ ███████████ ████████ ██ ████████ ████ ████ ██████████ ████ ███████ ███████ █████████ █████ ███████████ ███████ ████ ████ ████████████ ███████ ████ ██ ███ ████████
Phenomenon ·
Latitudinal biodiversity gradient
The phenomenon is that around the poles, there aren't that many species and as we move closer to the equator, i.e., as we move along the latitude or the different width bands of the earth towards the equator, we find more and more species.
P2
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Hypothesis 1 ·
Time theory
The tropics (closer to equator) had more time for species to emerge uninterrupted by ice ages.
Challenge to Hypothesis 1 ·
But arctics are also uninterrupted by ice ages
And not all temperate regions have been disrupted equally, yet they all tend to have fewer species than the tropics.
P3
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Hypothesis 2 ·
Species-energy
Equator (and tropics) gets more solar energy which leads to higher reproduction rates which leads to higher biomass which leads to more speciation.
Challenge to Hypothesis 2 ·
But these conditions aren't necessarily connected
High reproduction doesn't have to lead to high biomass if individuals have short life spans. High biomass doesn't have to lead to more speciation.
Challenge to Hypothesis 2 ·
Untested mechanism
The causal chain is speculative.
P4
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Hypothesis 3 ·
Climatic stability in tropics
Climatic stability means reliable supply of resources which means more species can tolerate overlap in their niches.
Challenge to Hypothesis 3 ·
Local ecology doesn't explain the latitudinal gradient
P5
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Hypothesis 4 ·
Regional speciation
Hypothesis is that speciation rates are higher towards the tropics and not negated by extinction rates. Hence, there's a net increase over time in the number of species. Author thinks this hypothesis is the "most plausible."
P6
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Mechanism for Hypothesis 4 ·
Isolation of population subgroups
New species form when isolated subgroups evolve separately. Subgroups in tropics more likely to survive and adapt into new species than in arctic.
Passage Style
Critique or debate
Phenomenon-hypothesis (RC)
20.
With which one of the █████████ ██████████ ██████████ ████████ ████████████ ███ ███ ███████████ ████████ ██ ██████ ██ ███████ █████ ███ ██████ ██ ████ ██████ ██ ██████
a
The time theory ██ ███ █████ █████████ ██ ████████ ███████████ █████ ██ ████ ███ █████████ ██████ ███ ██████ ██ ███ ████ ████ ████████ ███████████
The author does not rank which theory is the “least” plausible. Perhaps the other theories are equally implausible.
b
The rate-of-speciation hypothesis █████████ █ █████████ █████████ ██ ███ ██████████████████ ███████████
Supported, because the only objection the author brought up to the climatic stability hypothesis was that it doesn’t account for the origin of a higher regional pool of species in the tropics (see end of P4). Rate-of-speciation addresses this objection by proposing a way that the tropics would end up with a greater diversity of species.
c
The major objection ██ ███ ████ ██████ ██ ████ ██ ████ ███ ██████████ ███████ ███ ██████ ██ █████ ███ ███████████ ████████ ███████ ██████████ ████ ████████████ ███████ ███ █████ ████ ████████
Not supported, because the author doesn’t bring up this complaint when criticizing time theory (see P2).
d
Despite the claims ██ ███ ██████████████ ███████████ █ ████ ████ ██ ██████████ ██████ ███ ████████████ ██ ████ ██████ ██ █████ ████ ███ ███████ ████ ████ ████ ████████
Not supported, because although the author does believe high production rates can exist with low biomass, the author doesn’t suggest that high production rates are “more likely” with low biomass than with high.
e
An important advantage ██ ███ ██████████████████ ██████ ██ ████ ██ █████████ ███████ ███████████ ██ █ ████████ ██████ ████ █████ ████████
Not supported, because competition for resources isn’t an aspect of the rate-of-speciation theory (see the last two paragraphs).