Speciation: An Illustrated Introduction — Transcript

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1. 0:03[Music]
2. 0:13There are almost 10,000 known bird species on the planet.
3. 0:18How did they evolve into so many unique forms?
4. 0:22Take the Birds-of-Paradise for example, with their
5. 0:26dizzying array of colors, shapes, and sizes.
6. 0:29From genetic evidence, we know that the
7. 0:33ancestor of all 39 species was a plain
8. 0:37crow-like bird.
9. 0:40And the process that caused this one species to split into many-
10. 0:44is called speciation. Before we explain
11. 0:49speciation, let’s talk about what we mean when we use the word
12. 0:53species. According to the classic definition,
13. 0:57species are groups of actually or
14. 1:01potentially interbreeding natural populations
15. 1:04which are reproductively isolated from other groups.
16. 1:08Speciation begins
17. 1:11when groups become separated in space or
18. 1:14become different enough in form and behavior that individuals from one group
19. 1:19no longer regularly mate with individuals outside the group.
20. 1:23One way for a new species to evolve starts when a few individuals
21. 1:31colonize a new area that is outside their native range.
22. 1:35Take for example a volcanic
23. 1:39island that rises out of the sea just off the coast
24. 1:43of the mainland.
25. 1:46Over millions of years, as the island grows larger
26. 1:50and less volcanically active, plants begin to grow
27. 1:54but there are no birds here yet.
28. 1:58Then one day, birds from a common species on the mainland
29. 2:01are blown over by a storm and start a new population.
30. 2:05If these birds move between the island and the mainland
31. 2:10only rarely,
32. 2:11the conditions for reproductive isolation are set
33. 2:15and the brand new island population is on its way to becoming a new species.
34. 2:19Over thousands of generations
35. 2:24the two populations will be shaped by natural
36. 2:27and sexual selection into separate forms,
37. 2:30each with a unique genetic fingerprint.
38. 2:35But at what point are we confident that this
39. 2:39island population has really changed enough to become
40. 2:42a new species? Well let's put
41. 2:47our definition to the test. If we introduce a female
42. 2:51from the mainland species to potential mate from the island
43. 2:55after two hundred generations and isolation
44. 2:58(about four hundred years in bird terms), will they mate?
45. 3:03It appears that these two [chirp, chirp] still recognize each other as members of the
46. 3:07same species.
47. 3:08How about after another thousand generations of isolation?
48. 3:14Now the female finds this island male’s song a bit strange, [chirp, chirp]
49. 3:19but she still chooses to meet with him.
50. 3:22What if we fast-forward again to 10,000 generations of isolation? [tweetie, tweet]
51. 3:28This time
52. 3:33the female doesn't even recognize the male’s song and is completely
53. 3:37uninterested.
54. 3:41If this female’s mating preferences are widespread in the population,
55. 3:45we now have two bona fide species:
56. 3:48Mainland and Island.
57. 3:55But these kinds of dating match ups don’t happen in reality.
58. 3:59Here because our birds avoid flights across the open ocean,
59. 4:03Island birds do not regularly encounter Mainland birds.
60. 4:06Because genes from the two populations no longer intermix,
61. 4:12genetic differences accumulate and the two populations become
62. 4:17different species. So in practice,
63. 4:20scientists classify species separated in space
64. 4:24by their genetic differences, not by observing blind dates.
65. 4:28[Music]
66. 4:31But there is another kind of speciation
67. 4:35that can occur when a habitat becomes divided
68. 4:38and a once widespread bird population
69. 4:41becomes fragmented. Imagine that our new
70. 4:46island grows larger over time and the volcano develops
71. 4:50a central mountain ridge while a valley forms along the Eastern Coast.
72. 4:55Over time,
73. 4:58the Eastern birds who have plentiful fruit resources and only need a single
74. 5:03parent to successfully raise young
75. 5:05evolve specialized mating systems. Males compete for multiple mates, and
76. 5:11and to get noticed evolve fancy plumage
77. 5:15and display behaviors. On one side of the valley the males evolve elaborate
78. 5:19head plumes
79. 5:20and on the other it's the tales that get fancy.
80. 5:24At the same time Western birds are consistently dealing with harsher food
81. 5:30conditions
82. 5:31and do not evolve a specialized mating system
83. 5:34or fancy feathers. Just as before
84. 5:39early in the speciation process when individuals from
85. 5:43isolated populations meet, our Eastern and Western birds
86. 5:47readily mate. But as generations pass
87. 5:51and Eastern males become fancier, the Western females begin to find the
88. 5:56Eastern males’ habits foreign and surprising.
89. 6:00But these traits are clearly not deal breakers!
90. 6:04Even after 10,000 generations of geographic isolation,
91. 6:09some other Western females continue to find the male displays
92. 6:14attractive.
93. 6:18At this stage the eggs she lays aren’t
94. 6:21viable because the separated populations
95. 6:24have evolved too many genetic differences to be compatible.
96. 6:28To review, in this kind of speciation,
97. 6:33geographic barriers have split the habitat
98. 6:36into three fragments and on each a unique species
99. 6:41evolved.
100. 6:45What if the Southwestern slope
101. 6:47erodes into a low peninsula, allowing Southeastern birds to mix with the
102. 6:52Western birds?
103. 6:53By this time, their mating systems
104. 6:57are completely different and the two species can't create fertile
105. 7:02offspring. Now related species can coexist,
106. 7:07but because they no longer successfully interbreed,
107. 7:11there's no going back. So how did the 39 bird-of-paradise species
108. 7:21evolve from one crow-like ancestor? New Guinea
109. 7:27and the surrounding islands where birds- of-paradise evolved
110. 7:30have changed dramatically throughout geologic time,
111. 7:33repeatedly isolating populations.
112. 7:36Over thousands of generations in
113. 7:40isolation, natural and sexual selection
114. 7:44have morphed the fragmented populations into a wonderful array of
115. 7:48colorful species. The process of speciation is ongoing
116. 7:54and split after split
117. 7:56new and unique organisms evolve.
118. 8:01Incredibly, all those 10,000 bird species we see today
119. 8:05evolved from one single bird ancestor. And it’s thanks to the
120. 8:10evolutionary processes driving speciation
121. 8:13that our planet is home to such impressive
122. 8:17biodiversity.