ACT科学Session1生物（实战练习Practice 6）

　　(61F-Passage I)

　　Frog eggs develop into tadpoles, which then metamorphose into adult frogs. Recently, reported observations of frogs with morphological abnormalities (usually a missing or malformed hind limb) have increased in many localities. Four hypotheses attempt to explain this increase.

　　Hypothesis 1假设1

　　Tadpoles are commonly parasitized by trematodes, flatworms that burrow into the tadpoles, causing cysts to form. The presence of cysts during tadpole development can cause morphological abnormalities in the adult frog. Thus, increases in trematode populations most likely account for the recent increase in morphological abnormalities.

　　Hypothesis 2假设2

　　Due to industrial pollution, Earth’s ozone layer has thinned. This process has led to an increase in the amount of ultraviolet (UV) radiation reaching Earth’s surface. UV radiation can directly cause abnormalities in frog eggs by producing mutations in the DNA that controls frog development UV radiation can also indirectly cause abnormalities in frog eggs by chemically transforming certain pollutants into teratogens, chemicals that cause developmental abnormalities. Hypothesis 3假设3

　　Certain pesticides contain a class of compounds called retinoids. Frogs naturally produce retinoids, but at high concentrations, retinoids cause mutations in frog eggs. These mutations later result in morphological abnormalities. Many of these mutations affect genes that control hind limb development. Thus, the recent increase in morphological abnormalities is probably due to an increase in retinoids from pesticides.

　　Hypothesis 4假设4

　　Tadpoles and frogs are prey for many aquatic predators, including fish, turtles, and wading birds such as herons. Predator attacks are not always completely successful; many result in injured frogs and tadpoles. Permanent injuries in tadpoles and frogs could later appear to be developmental abnormalities. Thus, increases in predator populations most likely account for the recent increase in morphological abnormalities.

　　Practice 6.1

　　Which 2 hypotheses argue that the recent increase in morphological abnormalities in frogs is the result of human activities?

　　A. Hypotheses 1 and 4

　　B. Hypotheses 2 and 3

　　C. Hypotheses 2 and 4

　　D. Hypotheses 3 and 4

　　Practice 6.2

　　All 4 hypotheses propose that the factors causing morphological abnormalities in frogs are present in the:

　　F. pesticides used by farmers.

　　G. environment in which the frogs live.

　　H.UV radiation produced by the Sun.

　　J. bodies of the organisms that prey on frogs.

　　Practice 6.3

　　Which 2 hypotheses propose that direct contact between tadpoles and another organism results in morphological abnormalities in frogs?

　　A. Hypotheses 1 and 4

　　B. Hypotheses 2 and 3

　　C. Hypotheses 2 and 4

　　D. Hypotheses 3 and 4

　　Practice 6.4

　　An artificial pond was created. The following were added to the pond: trematodes, UV-induced teratogens, retinoids, frog predators, tadpoles, and materials necessary for tadpole development. The frequency of morphological abnormalities in frogs was very high. This finding best supports which, if any, of the 4 hypotheses, and why?

　　F. Hypothesis 1, since it shows that the morphological abnormalities are caused by parasitism by trematodes.

　　G. Hypothesis 2, since it shows that the morphological abnormalities are the result of DNA mutations caused by teratogens.

　　H. Hypothesis 3, since it shows that the morphological abnormalities are caused by the presence of retinoids.

　　J. None of the 4 hypotheses is best supported, because the result does not indicate which factor caused the morphological abnormalities.

　　Practice 6.5

　　Normal adult frogs were captured, tagged, and released into a pond. After 2 months the frogs were recaptured and examined. Five percent of these frogs showed morphological abnormalities, such as missing hind limbs. These results best support which hypothesis?

　　A. Hypothesis 1

　　B. Hypothesis 2

　　C. Hypothesis 3

　　D. Hypothesis 4

　　Practice 6.6

　　Supporters of Hypothesis 4 would most likely argue that the frequency of morphological abnormalities in frogs would be lowest in which of the following types of aquatic communities?

　　F. Those with fish, turtles, and wading birds

　　G. Those with fish and turtles, but without wading birds

　　H. Those with fish and wading birds, but without turtles.

　　J. Those without fish, turtles, and wading birds

　　Practice 6.7

　　A biologist has argued that some of the factors cited in both Hypothesis 1 and Hypothesis 2 are combining to cause the increase in the morphological abnormalities in frogs. Which of the following findings about trematodes, if true, would best support this view?

　　A. Trematodes are highly sensitive to injury from UV light

　　B. Trematodes are immobilized by teratogens.

　　C. Exposure to UV light increases trematode populations.

　　D. Exposure to UV light improves frogs’ resistance to trematode parasitism.

　　Answers for Practice 17-32: Practice 6 B G A J D J C

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