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OFFICIAL10 What does DNA evidence indicate about relationships among whales?

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[00:00.00]]Narrator: Listen to part of a lecture in a marine biology class.
[00:10.50]FEMALE PROFESSOR: We know whales are mammals and that they evolved from land creatures. [00:15.54]So the mystery is figuring out how they became ocean dwellers. [00:20.45]Because, until recently, there was no fossil record of what we call “the missing link,” that is, evidence of species that show the transition between land-dwelling mammals and today’s whales.[00:32.37]Fortunately, some recent fossil discoveries have made the picture a little bit clearer.
[00:38.16]For example, a few years back, in Pakistan, they found the skull of a wolflike creature; [00:44.70]it was about 50 million years old. [00:46.94]Scientists’d seen this wolflike creature before, but this skull was different; [00:52.49]the ear area of the skull had characteristics seen only in aquatic mammals, specifically whales. [01:00.00]Uh, then—also in Pakistan—they found the fossil of another creature, which we call Ambulocetus natans.
[01:10.14]That’s a mouthful, eh? [01:13.06]The name Ambulocetus natans comes from Latin, of course, and means “walking whale that swims”; [01:21.48]it clearly had four limbs that could have been used for walking. [01:25.84]It also had a long, thin tail typical of mammals, something we don’t see in today’s whales. [01:32.92]But: it also had a long skeletal structure, [01:36.82]and that long skeletal structure suggests that it was aquatic …
[01:41.69]And very recently, in Egypt, they found a skeleton of Basilosaurus. [01:48.68]Basilosaurus was a creature that we had already known about for over 100 years, [01:54.29]and it has been linked to modern whales because of its long whalelike body, [01:58.94]but this new fossil find showed a full set of leg bones, something we didn’t have before.
[02:05.62]The legs were too small to be useful—[02:07.92]they weren’t even connected to its pelvis and couldn’t have supported its weight—[02:12.21]but it clearly shows Basilosaurus’ evolution from land creatures, [02:16.52]so that’s a giant step in the right direction. [02:19.23]Even better, it establishes Ambulocetus as a clear link between the wolflike creature and Basilosaurus.
[02:26.81]Now, these discoveries don’t completely solve the mystery. [02:30.74]I mean, Ambulocetus is a mammal that shows a sort of bridge between walking on land and swimming, [02:37.20]but it also is very different from the whales we know today. [02:41.10]So really, we are working with just a few pieces of a big puzzle.
[02:47.01]Uh, a related debate involves some recent DNA studies. [02:53.38]Remember, DNA is the genetic code for any organism, [02:56.82]and when the DNA from two different species is similar, it suggests that those two species are related … [03:03.35]And when we compared some whale DNA with DNA from some other species, we got quite a surprise: [03:10.88]the DNA suggests that whales are descendants of the hippopotamus![03:15.84]Yes, the hippopotamus!
[03:17.56]Well, that came as a bit of a shock, [03:19.24]I mean, that a four-legged land and river dweller could be the evolutionary source of a completely aquatic creature up to 25 times its size? [03:28.69]Unfortunately, this revelation about the hippopotamus apparently contradicts the fossil record, which suggests that the hippopotamus is only a very distant relative of the whale, not an ancestor, [03:41.81]and, of course, as I mentioned, that whales are descended not from hippos but from that distant wolflike creature.
[03:48.77]So we have contradictory evidence. [03:51.63]And more research might just raise more questions and create more controversies. [03:56.70]At any rate, we have a choice: [03:58.72]we can believe the molecular data, the DNA, or we can believe the skeleton trail, but, unfortunately, probably not both …
[04:08.49]Uh, and there have been some other interesting findings from DNA research: [04:14.88]For a long time, we assumed that all whales that had teeth, including sperm whales and killer whales, were closely related to one another. [04:23.45]And the same for the toothless whales, like the blue whale and other baleen whales—we assumed that they’d be closely related.
[04:31.13]But recent DNA studies suggest that that’s not the case at all; [04:34.99]the sperm whale is actually closely related to the baleen whale, and it’s only distantly related to the toothed whales. [04:42.57]So that was a real surprise to all of us.

6.What does DNA evidence indicate about relationships among whales?

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正确答案:C
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教授提到 for the toothless whales, like the blue whale and other baleen whales, we assumed that they be closely related. But recent DNA studies suggest that that’s not the case at all.说明有牙齿的鲸鱼在DNA上并不一定相近。所以答案应该选C

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