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)
18.
Which one of the following, ██ █████ ████ ███████ ███████ ███ ██████████████████ ██████████ ██ ██ ██ █████████ ██ ███ ████████
a
A remote subgroup ██ █ ████████ ███████ ██ ████████ ████ ███ ████████ ██████████ ███ ██████ ██████ ██ ██████████ ████ ███████ ██ ████ ████████ ███████████
This is consistent with the hypothesis – subgroups can turn into new species and be unable to breed with original populations.
b
Investigation of a █████ ████ ██ █ ████████ ████ ██████ ███████ ████ ████ █████████ ███████ ███ ████ ██ ███████ ██ ███ ████ █████ ██ ██████████
This doesn’t weaken the argument, because the ability of species to coexist on the same resources isn’t relevant to rate-of-speciation; it’s relevant to the climatic stability hypothesis described in P4.
c
A correlation between ██████ ██████ ███████ ██████ ████████████ ███ █████ ███████████ ██ █████ ██████████ ██ ████████████ ████████████
This doesn’t weaken the argument, because higher energy isn’t relevant to the rate-of-speciation hypothesis; it’s relevant to the species-energy hypothesis in P3.
d
Researchers find more ████████████ ███████ ██████ ██ █████████████ ██ ██ ██████ ██████ ████ ████ ███ ████████████
This doesn’t weaken the argument, because the rate-of-speciation hypothesis doesn’t suggest that there wouldn’t be some undiscovered arctic species. Maybe there are undiscovered species all over the world; what matters is whether rate-of-speciation explains why there are more species in the tropics.
e
Most of the ████████ █████████ ██ █████████ ████ ██████ █ ████████ ████ ███ █████ ██ ██████████ █████ ███████████
This weakens the argument, because the hypothesis holds that subgroups should involve a much lower rate of extinction than subgroups in the arctic. Further, the rate of extinction in the tropics shouldn’t be so fast that it negates the speciation rates.