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 variability is an incidental expression of something more adaptively significant. Indeed, the observed relationship between cerumen and odorous bodily secretions, to which non-human primates and, to a lesser extent humans, pay attention suggests that during the course of human evolution genes affecting body secretions, including cerumen, came under selective influence.
Q.1 It can be inferred from the passage that human populations vary considerably in their
A) ability to assimilate artificial chemicals
B) vulnerability to certain toxins found in plants
C) ability to discern bitterness in taste
D) sensitivity to certain bodily odors
Q.2 Which of the following provides the most reasonable explanation for the assertion in the first paragraph that evolutionary history "obviously" did not act on PTC?
A) PTC is not a naturally occurring chemical but rather has been produced only recently by scientists.
B) Most humans lack sufficient taste sensitivity to discriminate between PTC and bitter chemicals occurring naturally.
C) Variability among humans respecting PTC discrimination, like variability respecting earwax, cannot be explained in terms of evolutionary adaptivity.
D) Unlike non-human primates, humans can discriminate intellectually between toxic and non-toxic bitter substances.
Q.3 Which of the following best expresses the main idea of the passage?
A) Artificially synthesized chemicals might eventually serve to alter the course of evolution by desensitizing humans to certain tastes and odors.
B) Some human polymorphisms might be explained as vestigial evidence of evolutionary adaptations that still serve vital purposes in other primates.
C) Sensitivity to taste and to odors have been subject to far greater natural selectivity during the evolution of primates than previously thought.
D) Polymorphism among human populations varies considerably from region to region throughout the world.
Q.4 It can be inferred from the passage that
A) The amount of bodily odours and secretion that take place reduce at each stage of evolution.
B) The extent of attention paid by non-human primates to body secretions is much higher than that of the more evolved human species.
C) Artificially sythesized chemicals have impaired the extent of sensitivity that human beings have to body secretions.
D) All of these
1) Ans. C
2) Ans. A
3) Ans. B
4) Ans. B