2024年3月12日发(作者：)

Exercise 13

Biologists have long maintained that two groups of

pinnipeds, sea lions and walruses, are descended from

a terrestrial bearlike animal, whereas the remaining

group, seals, shares an ancestor with weasels. But the

5 recent discovery of detailed similarities in the skeletal

structure of the flippers in all three groups undermines

the attempt to explain away superficial resemblance as

due to convergent evolution—the independent develop-

ment of similarities between unrelated groups in

10 response to similar environmental pressures. Flippers

may indeed be a necessary response to aquatic life,

turtles, whales, and dugongs also have them. But the

common detailed design found among the pinnipeds

probably indicates a common ancestor. Moreover,

15 walruses and seals drive themselves through the water

with thrusts of their hind flippers, but sea lions use

their front flippers. If anatomical similarity in the

flippers resulted from similar environmental pressures,

as posited by the convergent-evolution theory, one

20 would expect walruses and seals, but not seals and sea

lions, to have similar flippers. (162 words)

1. The author implies that which of the following was part

of the long-standing view concerning pinnipeds?

□A Pinnipeds are all descended from a terrestrial

bearlike animal.

□B Pinnipeds share a common ancestor with turtles,

whales, and dugongs

□C Similari