新GRE阅读理解36套每日讨论--32（群178674184）

普渡哥

Flatfish, such as the flounder, are among the few
   vertebrates that lack approximate bilateral symmetry
   (symmetry in which structures to the left and right of the
   body‘s midline are mirror images). Most striking among
5 the many asymmetries evident in an adult flatfish is eye
   placement: before maturity one eye migrates, so that in
   an adult flatfish both eyes are on the same side of the head.
   While in most species with asymmetries virtually
   all adults share the same asymmetry, members of the
10 starry flounder species can be either left-eyed
   (both eyes on the left side of head) or right-eyed.
   In the waters between the United States and Japan,
   the starry flounder populations vary from about 50 percent
   left-eyed off the United States West Coast, through about 70 percent
15 left-eyed halfway between the United States and Japan, to
   nearly 100 percent left-eyed off the Japanese coast.
   Biologists call this kind of gradual variation over a certain
   geographic rang a “cline" and interpret clines as strong
   indications that the variation is adaptive, a
20 response to environmental differences. For the
   starry flounder this interpretation implies that a
   geometric difference (between fish that are mirror
   images of one another) is adaptive, that left-eyedness
   in the Japanese starry flounder has been selected for, which provokes a
25 perplexing questions: what is the selective advantage in
   having both eyes on one side rather than on the other?
   The ease with which a fish can reverse the effect of the sidedness
   of its eye asymmetry simply by turning
   around has caused biologists to study internal anatomy,
30 especially the optic nerves, for the answer. In all flatfish
   the optic nerves cross, so that the right optic nerve is joined
   to the brain‘s left side and vice versa. This crossing introduces
   an asymmetry, as one optic nerve must
   cross above or below the other. G. H. Parker
35 reasoned that if, for example, a flatfish‘s left eye
   migrated when the right optic nerve was on top, there
   would be a twisting of nerves, which might be
   mechanically disadvantageous. For starry flounders,
   then, the left-eyed variety would be selected against, since in a
40 starry flounder the left optic nerve is uppermost.
   The problem with the above explanation is that the
   Japanese starry flounder population is almost exclusively
   left-eyed, and natural selection never promotes a purely
   less advantageous variation. As other explanations
45 proved equally untenable, biologists concluded that
   there is no important adaptive difference between
   left- eyedness and right-eyedness, and that the
   two characteristics are genetically associated with some
   other adaptively significant characteristic. This
50 situation is one commonly encountered by
   evolutionary biologists, who must often decide
   whether a characteristic is adaptive or selectively neutral.
   As for the left-eyed and right-eyed flatfish,
   their difference, however striking, appears to be an
55 evolutionary red herring.


(456 words)



10. According to the passage, starry flounder differ
from most other species of flatfish in that starry
flounder
(A) are not basically bilaterally symmetric
(B) do not become asymmetric until adulthood
(C) do not all share the same asymmetry
(D) have both eyes on the same side of the head
(E) tend to cluster in only certain geographic regions


For the following question, consider each of the choices separately and select all that apply
11. The author would be most likely to agree
with which of the following statements
about left-eyedness and right-eyedness in
the starry flounder?
□A They are adaptive variations by the starry flounder
to environmental differences.
□B They do not seem to give obvious selective
advantages to the starry flounder.
□C They occur in different proportions in different
locations.



12. According to the passage, a possible disadvantage
associated with eye migration in flatfish is that the
optic nerves can
(A) adhere to one another
(B) detach from the eyes
(C) cross
(D) stretch
(E) twist


13. Which of the following best describes the
organization of the passage as a whole?
(A) A phenomenon is described and an
interpretation presented and rejected.
(B) A generalization is made and supporting
evidence is supplied and weighed.
(C) A contradiction is noted and a resolution is
suggested and then modified.
(D) A series of observations is presented
and explained in terms of the dominant theory.
(E) A hypothesis is introduced and corroborated
in the light of new evidence.

jz20000cn

没人做？

元宝盒子

C
BC
E
B

普渡哥

最后一个错了，继续加油！

bennyaaron

c
ac
e
a

bennyaaron

c
Bc
e
a
好了，我改一下

普渡哥

对了。
大家加油复习啊