問題詳情

III. Reading Comprehension (20%)    Each year, scientists publish roughly 17,000 detailed descriptions of newly discovered animals. Recently, in the journalBreviora, researchers described yet another, a new species of lizard called Aspidoscelis neavesi.    At first glance, this seems to be a run-of-the mill lizard: a small, slender creature with spots along its back and a bluish tail.In fact, Aspidoscelis neavesi is quite exceptional. The lizard was produced in the laboratory by mating two other species, and itscreation defies conventional ideas about how new species evolve.    The evolution of a new animal species is usually a drawn-out affair. Typically, an existing animal population is somehowdivided, and the newly isolated populations reproduce only among themselves. Over thousands of generations, the animals maybecome genetically distinct and can no longer interbreed.    Of course, scientists have long known that some related species sometimes interbreed. But the hybrid progeny generallywere thought to be evolutionary dead-ends — sterile mules, for instance. In recent decades, however, researchers have learnedthat these hybrids may represent new species.      Some of the most striking examples occur among whiptail lizards, which live in the southwestern United States. In the1960s, scientists noticed that some whiptail lizard species had a strange genetic makeup. They have two copies of eachchromosome, just as we do, but each copy is very different from its counterpart. The genes look as if they come from differentspecies. Perhaps stranger, many species produce no males. The eggs of the females hatch healthy female clones, a processknown as parthenogenesis.    Normally, unfertilized animal eggs have only one set of chromosomes. The second set is derived from a male’s spermfollowing fertilization. But parthenogenic female whiptail lizards can duplicate the chromosomes in their offspring withoutmales. These findings led scientists to a hypothesis for how these strange species came about: Sometimes individuals from twodifferent species of whiptail lizards interbreed, and their hybrid offspring carry two different sets of chromosomes.    Somehow, this triggers a switch to parthenogenesis. The female hybrids start to produce clones distinct from eitherparental species. In other words, they instantly become a new species of their own.Dr. Neaves didn’t follow up on this finding, instead pursuing a career researching fertility and stem cells. But at a dinner in2002, he mentioned the whiptail lizards to Peter Baumann, a molecular biologist at Stowers Institute for Medical Research,where Dr. Neaves served as president.    Dr. Baumann decided it was high time to use new scientific tools to study whiptail lizards, and he and Dr. Neaves startedmaking road trips to New Mexico to catch them and take them back to Stowers. As they came to understand the biology of thelizards better, they and their colleagues began to bring different species together to see if they could hybridize. Most of the time,their experiments failed.     In 2008, the scientists tried to recreate the hybrid with four sets of chromosomes. They put female Aspidoscelis exsanguis(the parthenogenic species with three sets of chromosomes) and male Aspidoscelis inornata in the same containers. In shortorder, the lizards started mating, and the females laid eggs. When the eggs hatched, the scientists examined the genes of thebaby lizards and found four sets of chromosomes.    Four of the new hybrids were females. To the delight of the scientists, the females could clone themselves — and theoffspring could produce clones of their own. Today, the scientists have a colony of 200 of these lizards.
21. Why does the author mention “sterile mules” in the fourth paragraph?
(A) To delineate one of the only instances of an occurrence.
(B) To contradict the opinion presented in the passage.
(C) To provide evidence that supports scientists’ beliefs.
(D) To reiterate that the lizard is an unusual creature.

參考答案

答案：C
難度：適中0.5
統計：A(0),B(0),C(0),D(0),E(0)

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