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.
Questions
- It can be inferred from the passage that human
populations vary considerably in their
- sensitivity to certain bodily odors
- ability to assimilate artificial chemicals
- vulnerability to certain toxins found in
plants
- ability to discern bitterness in taste
- 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?
- PTC is not a naturally occurring chemical but
rather has been produced only recently by scientists.
- Most humans lack sufficient taste sensitivity
to discriminate between PTC and bitter chemicals occurring naturally.
- Variability among humans respecting PTC
discrimination, like variability respecting earwax, cannot be explained
in terms of evolutionary adaptivity.
- Unlike non-human primates, humans can
discriminate intellectually between toxic and non-toxic bitter
substances.
- Which of the following best expresses the main
idea of the passage?
- Artificially synthesized chemicals might
eventually serve to alter the course of evolution by desensitizing humans
to certain tastes and odors.
- Some human polymorphisms might be explained
as vestigial evidence of evolutionary adaptations that still serve vital
purposes in other primates.
- Sensitivity to taste and to odors have been
subject to far greater natural selectivity during the evolution of
primates than previously thought.
- Polymorphism among human populations varies
considerably from region to region throughout the world.
- It can be inferred from the passage that
- The amount of bodily odours and secretion
that take place reduce at each stage of evolution.
- The extent of attention paid by non-human
primates to body secretions is much higher than that of the more evolved
human species.
- Artificially sythesized chemicals have
impaired the extent of sensitivity that human beings have to body
secretions.
- All of these
Solutions:
- 4
- 1
- 2
- 2
Explanations
1. The doubt would have been between Answer choice
(1) and (4). While the passage states about the amount of odorous bodily
secretion the Answer choice (1) talks about the sensitivity to such
secretions.
3. Choice (3) talks about sensitivity to odors, while the passage talks about the secretions and not the sensitivity to them. Choice (4) talks about regional variation to polymorphism as the central idea of the passage, while the central idea is about polymorphism in general and how it has changed with evolution.
3. Choice (3) talks about sensitivity to odors, while the passage talks about the secretions and not the sensitivity to them. Choice (4) talks about regional variation to polymorphism as the central idea of the passage, while the central idea is about polymorphism in general and how it has changed with evolution.
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