物种分化_Speciation

[00:01.55]Narrator:Listen to part of a lecture in a Biology class.
[00:08.08]Professor:What makes one species separate into two different species? [00:13.44]And, how long does it take? [00:15.60]Biologists are always intrigued by evolutionary processes that give rise to new plants and animal species. [00:24.10] Or, speciation. [00:25.92]One cause of speciation is when a barrier divides a population of organisms into two groups. [00:33.46]Barriers either directly or indirectly interrupt the flow of genes within the population. [00:40.75]The genetic makeup of each group diverges, and they become less similar over time as each group adapts to its particular environment. [00:51.07]At some point, we may have two related but genetically distinct subpopulations, and eventually the differences may be sufficient to create a new species. [01:04.13]Most barriers that are physical barriers. [01:07.55]These could be natural barriers, like rivers, or human made ones. [01:12.72]A good example of a human made barrier is the Great Wall of China. [01:18.05]Um, there was recently a study of subpopulations of plants growing wild on either side of the Great Wall. [01:26.55]And it showed that genetic differences do exist between sub-populations. [01:32.55]So the Great Wall is acting as a barrier to gene flow. 
[01:37.44]Student:Are the differences enough to consider the plants different species?
[01:41.95]Professor:Not yet, but the process has started. [01:45.41]For a few more centuries, enough differences may arrive to consider these different species. [01:52.52]Okay, but barriers to gene flow need not by physical. [01:57.41]In animals behavioral barriers can interrupt gene flow and speciation. [02:03.31]Uh- consider bird song. [02:06.06]Each species of songbird communicates through a specific pattern of tweets, whistles and thrills. [02:13.49]It - it- it it’s like, their language, their signal. [02:18.66]The songs trigger behaviors necessary for birds survival, like mating and defense. [02:25.63]So birds may not respond to the songs of other species. [02:29.81]But bird songs aren’t static, they change over time; they evolve. [02:35.23]Several studies have shown that there are geographic variations even within the song of the same species. [02:43.34]Some have actually developed local accents or dialect. [02:47.88]And they don’t respond to a bird of their own species singing in a different dialect. [02:54.20]So this is one way that bird song can become a barrier to gene flow. [02:59.40]One that’s just as effective as a physical barrier. [03:03.60]Now, a related but maybe more interesting question is, does bird song evolve over time as well as over distance? [03:13.25]Some research into this question has been conducted by Elizabeth DerryBerry. [03:19.33]DerryBerry’s research was possible because people have been making high quality recordings of birdsong, since the 1960s. [03:28.85]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. [03:39.99]First, she compared the sparrow’s song from 1979 to a more modern version, one recorded in 2003. [03:48.96]Derryberry found that the songs were the same in most respects. [03:53.77]Where they differed was in pace and pitch. [03:56.83]The modern song is slightly slower and lower than the historical song. [04:01.77]Next, Derryberry put female sparrows in cages, played the historical and modern songs, and observed the birds reactions. [04:12.12]Um, typically the females respond by doing things like arching their backs and lifting their beaks to show interest in mating. [04:21.12]In the cage experiment, these mating behaviors were far more pronounced in response to the modern version of the song versus the historical version. [04:31.77]Since it’s the males who sing, Derryberry conducted a different experiment with them. [04:38.47]Male birds are territorial, so they will defend their territory against intrusion from other males. [04:46.25]When a male bird hears arrival, another male singing his species mating song, he will aggressively approach the intruders to scare him away. [04:57.14]So it was important to study the male sparrows in their natural environment. [05:02.38]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. [05:14.42]The males were far more aggressive, they approached the speaker more closely, when the modern song was played.
[05:22.88]Student:I don’t get it. [05:24.05]I mean, so birds today respond differently to old songs than they do to new songs. [05:30.48]But it’s not like some birds are still singing the old songs?  
[05:34.73]Professor:You’re right. [05:35.99]Uh - But look at it this way. [05:38.47]The Great Wall’s 100 of years old. [05:41.74]So those plant populations divided by the Great Wall have been accumulating differences for a long time. [05:49.80]And we still don’t have speciation. [05:52.73]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.