Self-study
P1
Until recently, biologists were unable to explain the fact that pathogens—disease-causing parasites—have evolved to incapacitate, and often overwhelm, their hosts. ████ ████████ ██ ██ ████ ████ ███ ██████████ ████ ██ █████████████ █████████████████ ██ ████████ ████ ███ ████████ ██████████ ███████ █ ██████ ████████████ ████ ████ ██ █████ ██ ███ ████ ████ █████████ ████ ██ ███ ████ █████ █████ ████ ███ ████ ██████ ███ █████████ █████████ ████ ██████ ███████ █████ █████ ███████ ███
Puzzling Phenomenon ·
Pathogens incapacitate and kill hosts
This is puzzling because the prevailing view is that parasites need their hosts for long-term survival.
Hypothesis ·
Trade-off
Pathogen can cause hosts to be very sick or even die if it can spread to a new host. So it's trading away staying in the current host for jumping to a new host.
P2
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Implication ·
Virulence is function of mode transmission
Virulence means the ability to defeat the host's defenses and incapacitate the host.
Example ·
Close contact transmission requires low virulence
Rhinoviruses (common cold) requires physical contact for transmission. Hence, it cannot be too virulent. If it were, then the host would be confined in solitude (or die) and the virus would not be able to jump to a new host. It needs to be only mildly virulent so that the host can help it jump to new hosts.
P3
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Example ·
Vector transmission requires high virulence
If a pathogen can be transmitted via a vector (e.g., mosquito), then virulence can and perhaps must be high. It needs to reproduce so extensively (and hence severely harm the host) such that the vector (e.g., mosquito) can acquire enough of the pathogen to transmit it to a new host.
P4
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Anomalies ·
"Sit and wait" pathogens
In general, the literature supports the hypothesis. But "sit and wait" pathogens like diphtheria and tuberculosis are directly transmitted yet lethal. They can do this because they can survive outside their hosts for up to months. This makes them the most dangerous kind of pathogen.
Passage Style
Phenomenon-hypothesis (RC)
Single position
24.
Which one of the following ████ ██████████ █████████ ███ ████████████ ██ ███ ████████
a
introduction of a ██████████ ████████ ████████████ ██ ██ ███████████ ███ ███ ████████ ███████ ██ ██ ███████████ ██ ███ ████████████ ██████████ ██ ███ ████████ ████████████ ███ ████████████ ██████████ ██ ██████████ ██ ███ ███████████
This is the best answer. The passage introduces a scientific anomaly (some pathogens incapacitate their hosts), presents an explanation for the anomaly (the biologists’ hypothesis), mentions an implication of the explanation (virulence depends on mode of transmission), discusses two examples showing the implication (rhinoviruses and the mosquito-borne pathogen), and discusses exceptions (”sit and wait” pathogens).
b
introduction of a ██████████ ████████ ████████████ ██ ██ ███████████ ███ ███ ████████ ██████████ ██ ███ ████████ ████████████ ███ ████████████ ██████████ ██ ██████████ ██ ███ ████████████ ███████ ██ ██ ███████████ ██ ███ ███████████
The easiest way to eliminate (B) is to notice the end — “mention of an implication of the explanation.” There is no implication of the explanation presented at the end. Rather, we get exceptions to an implication.
c
introduction of a ██████████ ████████ ████████████ ██ ██ ███████████ ███ ███ ████████ ██████████ ██ ███ ████████ ████████████ ███ ████████████ ███████ ██ ██ ███████████ ██ ███ ████████████ ██████████ ██ ████████ ████████████ ███ ███████████
The easiest way to eliminate (C) is to notice that it’s missing the exceptions discussed at the end of the passage. The passage does not finish by discussing “examples illustrating the implication.”
d
introduction of a ██████████ ████████ ████████████ ██ ██ ███████████ ██ ███ ████████ ██████████ ██ ███ ████████ ████████████ ███ ████████████ ██████████ ██ ██████████ ██ ███ ███████████
This misses the biologists’ hypothesis. The passage presents an explanation for the scientific anomaly before going into an implication of the explanation. Also, the implication is from the explanation, not from the anomaly itself.
e
introduction of a ██████████ ████████ ██████████ ██ ███ ████████ ████████████ ███ ████████ ████████████ ██ ██ ███████████ ███ ███ ████████ ██████████ ██ ████████ ████████████ ███ ███████████
The passage doesn’t present two examples of the anomaly after introducing the anomaly (the fact that some pathogens incapacitate their hosts). This also misses the exception to the implication discussed at the end.