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第1段
1 .listen to part of a lecture in a zoology class.
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2 .Until recently, our main approach to classifying organisms into groups, their species genus,
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3 .had been to classify them according to their similar physical features.
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4 .These classifications have helped us understand how organisms are related in terms of their evolution.
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5 .Of course, Recent breakthroughs in DNA analysis have given us new information about many organisms, causing us to go back and reclassify them.
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6 .But here\'s an interesting case where DNA analysis actually supported some previous classifications.
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7 .Okay, our story starts with Vladimir Nabokov.
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8 .Nabokov the author from Russia.
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9 .Yes, most of us know him for his fiction that made him famous in the 1950s right?
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10 .But little did you know he also did extensive work as a taxolimist
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11 .in particular, Navajo specialized in classifying species of butterflies.
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12 .Well, in 1945 he wrote an extensive research paper,
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13 .and that paper contained a radical hypothesis about a new way to classify a group of South American butterflies and its evolutionary origins.
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14 .The group of butterflies is called polyomatis blues, or blues for short.
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15 .So why radical?
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16 .Well, Blues have been studied for centuries, but there are over 400 species classified as blues,
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17 .and they can be found in most of the northern regions of the world, but also from Central Mexico to most of South America.
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18 .So their evolutionary relationships were far from understood.
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19 .And where did they originate? Nobody knew.
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20 .But in his paper, navakov proposed a new classification scheme for blues
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21 .and a very specific hypothesis for the evolution of North and South American blues
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22 .that they\'d all evolved from Asian ancestors.
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23 .This kind of detailed analysis simply was unheard of among experts at that time.
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24 .But how would Blues have gotten to the Americas, all the way from Asia?
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25 .Let\'s look at a world map.
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26 .Okay, Nabokov believed that the ancestor of blues had migrated from Asia into Alaska, crossing what\'s now the Bering Strait, which was a solid landmass at the time.
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27 .And this happened in five separate colonizing waves with a very specific timeline.
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28 .The first wave took place about 11 million years ago,
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29 .after reaching Alaska, this first group of blues slowly dispersed in North America and southwards down into South America,
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30 .evolving into the South American species we find there today.
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31 .They subsequently died out completely in North America
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32 .after that, between nine and 1 million years ago, four other distinct colonizing events originating in Asia occurred in succession,
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33 .again at very specific times.
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34 .Each colonization produced a new and distinct group of blues, each evolving from the ancestors that made the trip.
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35 .And each of these distinct groups remained in the northern regions after crossing the Bering Strait, evolving into the species we find throughout much of North America today,
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36 .some hypothesis,
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37 .but guess what?
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38 .The scientific community just dismissed Nabokov\'s ideas, mainly because he had no formal scientific training.
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39 .But fast forward to today.
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40 .Recently, another butterfly expert, Naomi Pierce,
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41 .that\'s P, I, E, R, C, E, Pierce.
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42 .She read Nabokov\'s paper,
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43 .and she became so intrigued with his hypothesis that she decided to test it out.
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44 .So she and her team did extensive work extracting and analyzing the DNA of a wide range of Asian and American blues species to determine their relationships.
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45 .And they concluded from their analysis that not only were Nabokov\'s classifications right, but that American blues could be traced back to a single common Asian ancestor.
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46 .Wow was Nabokov right about the timing of the migrations
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47 .he was and how did they show this?
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48 .Well, temperatures have fallen in the Bering Strait region over the last 11 million years, right?
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49 .So they looked at exactly when the temperatures fell and by how much during this period of time.
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50 .Then they studied the temperature tolerances and distribution of existing blues
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51 .This allowed them to estimate the temperature tolerances of blues ancestors,
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52 .and they concluded that as the temperatures in the area surrounding the Bering Strait dropped,
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53 .each new colonizing species had been more tolerant to the cold,
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54 .allowing them to make the trip across The Bering Strait,
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55 .oh, and the first group that thrived in warmer climates dispersed southward, is temperatures dropped, right.
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56 .Whereas the subsequent groups could tolerate the northern climates, so they evolved into the species we find in the northern areas of the Americas today.
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