TOEFL Listening: ETS-TOEFL听力机经 - 011ITJ4WH3R0CE14Q$

Lecture: Speciation: Narrator: Listen to part of a lecture in a biology class. Professor: What makes one species separate into two different species? And, how long does it take? Biologists are always intrigued by evolutionary processes that give rise to new plants and animal species. Or, speciation. One cause of speciation is when a barrier divides a population of organisms into two groups. Barriers either directly or indirectly interrupt the flow of genes within the population. The genetic makeup of each group diverges, and they become less similar over time as each group adapts to its particular environment. At some point, we may have two related but genetically distinct subpopulations, and eventually the differences may be sufficient to create a new species. Most barriers, strew flow, are physical barriers. These could be natural barriers, like rivers, or human made ones. A good example of a human made barrier is the Great Wall of China. Um, there was recently a study of subpopulations of plants growing wild on either side of the Great Wall. And it showed that genetic differences do exist between sub-populations. So the Great Wall IS acting as a barrier to gene flow. Student: Are the differences enough to consider the plants different species? Professor: Not yet, but the process has started. If the great wall stands for a few more centuries, enough differences may arrive to consider these different species. Okay, but barriers to gene flow need not by physical. In animals behavioral barriers can interrupt gene flow and lead to speciation. Uh ... consider bird song. Each species of songbird communicates through a specific pattern of tweets, whistles and thrills. It – it- it it's like, their language, their signal. The songs trigger behaviors necessary for birds survival, like mating and defense. So birds may not respond to the songs of other species. But bird songs aren't static, they change over time; they evolve. several studies have shown that there are geographic variations even within the song of the same species. Some have actually developed local accents or dialect. And they don't respond to a bird of their own species singing in a different dialect. So this is one way that bird song can become a barrier to gene flow. One that's just as effective as a physical barrier. Now, a related but maybe more interesting question is, does bird song evolve over time as well as over distance? Some research into this question has been conducted by Elizabeth DerryBerry. DerryBerry's research was possible because people have been making high quality recordings of birdsong, since the 1960s. Derryberry chose to study a population of white crown sparrows, mainly because it's a species whose songs have been recorded for over 40 years. First, she compared the sparrow's song from 1979 to a more modern version, one recorded in 2003. Derryberry found that the songs were the same in most respects. Where they differed was in pace and pitch. The modern song is slightly slower and lower than the historical song. Next, Derryberry put female sparrows in cages, played the historical and modern songs, and observed the birds reactions. Um, typically the females respond by doing things like arching their backs and lifting their beaks to show interest in mating. In the cage experiment, these mating behaviors were far more pronounced in response to the modern version of the song versus the historical version. Since it's the males who sing, Derryberry conducted a different experiment with them, male birds are territorial, so they will defend their territory against intrusion from other males, when a male bird hears arrival, another male singing his species mating song, he will aggressively approach the intruders to scare him away. So it was important to study the male sparrows in their natural environment. Derryberry set up speakers in the territories of several male sparrows, and observed how the birds behave in response to the modern and historical songs. The males were far more aggressive, they approached the speaker more closely, when the modern song was played. Student: I don't get it. I mean, so birds today respond differently to old songs than they do to new songs. But it's not like some birds are still singing the old songs? Professor: You're right. Uh – But look at it this way. The Great Wall's 100 of years old. So those plant populations divided by the Great Wall have been accumulating differences for a long time. And we still don't have speciation. But Derryberry's sparrow study hints that behavioral barriers to gene flow, well at least in birds, can evolve in a much shorter timespan than we've imagined.