Multiple choice

Which of the following best expresses the main idea of the passage?

Directions: Read the passage and answer the question.

In nearly all human populations a majority of individuals can taste the artificially synthesized chemical phenylthiocarbonide (PTC). However, the percentage varies dramatically – from as low as 60% in India to as high as 95% in Africa. That this polymorphism is observed in non - human primates as well indicates a long evolutionary history which although obviously not acting on PTC, might reflect evolutionary selection for taste discrimination of other, more significant bitter substances, such as certain toxic plants. A somewhat more puzzling human polymorphism is the genetic variability in earwax, or cerumen which is observed in two varieties. Among European populations 90% of individuals have a sticky yellow variety rather than a dry, gray one, whereas in northern China these numbers are approximately the reverse. Perhaps like PTC variability, cerumen is an incidental expression of something more adaptively significant. Indeed, the observed relationship between cerumen and odorous bodily secretions denote that during the course of human evolution, genes affecting body secretions, including cerumen, came under selective influence.

  1. Some human polymorphisms might be explained as vestigial of evolutionary adaptations that still serve vital purposes in other primates.

  2. Sensitivity to taste and to odors have been subject to far greater natural selection during the evolution of primates than previously thought.

  3. Polymorphism among human populations varies considerably from region to region throughout the world.

  4. The human senses of taste and smell have evolved considerably over the course of evolutionary history.
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A Correct answer
Explanation

In the first paragraph the author’s main concern is to point out that the variability among human populations regarding sensitivity to PTC might be a trace of the evolutionary process of natural selectivity. Then in the second paragraph the author offers a similar suggestion about variability in earwax type. To support these assertions the author refers that both characteristics still serve useful purposes among non-primates from whom humans presumably evolved. This inference is especially clear with respect to indentifying bitter substances that might be toxic. (a) accurately reflects the author’s main assertion and supporting evidence.

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