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# Under the model of natural selection, within populations that are

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Re: Under the model of natural selection, within populations that are [#permalink]
1 st question , why a not e...
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Re: Under the model of natural selection, within populations that are [#permalink]
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Kuanl wrote:
the answer to 1st question is a and to 2nd question is c are the ans right

ishitaz wrote:
Not sure where in the passage "external forces" was mentioned for Natural Selection

Official Explanation

2. Which of the following best describes the relationship between natural selection and genetic drift?

Difficulty Level: 700

Explanation

This question is asking you the difference between two specific terms defined in the text - natural selection and genetic drift. Based on the information given in the passage, natural selection deals with how the traits of a population change over time based on environmental pressures. (Note that there is no mention of population size mentioned when natural selection is discussed.) Genetic drift is described as the changes in a very small population that occur due to chance even if they are not necessarily advantageous changes. Genetic drift is discussed as something that happens in addition to natural selection, not instead of it.

The only answer that matches this is (C). Natural selection deals with changes due to external forces (environmental pressures), and genetic drift occurs alongside natural selection in small populations.

Among the other answers, choice (A) can be eliminated because natural selection does not occur only in large populations.

Choice (B) can be eliminated because both deal with the changes in the traits of a population over time - one is just due to chance and the other due to external pressures.

Choice (D) can be eliminated because genetic drift is important primarily in small populations, not large ones.

Choice (E) can be eliminated because genetic drift is described as a phenomenon separate from natural selection, not as a part of natural selection.

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Re: Under the model of natural selection, within populations that are [#permalink]
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GAngstA wrote:
1 st question , why a not e...

Official Explanation

1. Which of the following best describes the structure of the passage?

Difficulty Level: 600

Explanation

This question asks you about the structure of the passage, which generally is going to mean that you need to call upon your STOP reading of the passage to ensure that the answer you pick has the correct scope, tone, and main idea as the passage itself. This passage starts by introducing natural selection and explaining how it works before presenting an apparent paradox: why do genes that cause diseases persist if natural selection should get rid of them. The next paragraph then discusses a potential reason, genetic drift, which is given as a phenomenon that occurs alongside natural selection but that does not invalidate the idea of natural selection.

The only answer choice that matches this is (A). The passage introduces a model (natural selection), an apparent paradox (why deleterious genes exist), and then offers a complementary model to explain this paradox. Note that the word "complementary" indicates that the model of genetic drift does not replace natural selection, but rather adds to it.

Among the other answers, (B) and (C) can be eliminated because the theory of natural selection is neither discarded nor replaced by genetic drift.

Choice (D) can be eliminated because while the model of natural selection is improved upon by the addition of genetic drift, drift is described as a separate phenomenon.

Choice (E) can be eliminated because even though the passage does present an inherent paradox, the passage uses the example to resolve that paradox rather than weaken the original model.

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Re: Under the model of natural selection, within populations that are [#permalink]
Hi,
how can we infer that genetic drift is something that happens in addition to natural selection?
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Re: Under the model of natural selection, within populations that are [#permalink]
animeshdash1000 wrote:
Hi,
how can we infer that genetic drift is something that happens in addition to natural selection?

I believe this line in p/g 2 may have the answer: "For small populations, the laws of probability exhibit a stronger influence over evolution than the pressures of natural selection, although natural selection is still a factor."
Re: Under the model of natural selection, within populations that are [#permalink]
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