Bio171-F11-Lec 27 - Biology 171 Wednesday, November 9, 2011...

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Biology 171 Wednesday, November 9, 2011 Lecture 25: Phylogenetics and the Tree of Life Announcements This Week in Discussion: Review HWE; Evolution in Sticklebacks Exam 3 – Wed. Nov. 16 8-10 PM. Text Reading Lecture 25 : Chapter 27 (474-479) Lecture 26 : Chapter 27 (479-492) Speciation (concluded) Speciation by polyploidy Testing speciation Darwin’s Tree of Life Ancestral & Derived Characters Homology and Homoplasy Building Phylogenetic trees
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Sympatric speciation Via Polyploidy (increase in the number of complete sets of chromosomes) Very common in plants - less so in animals Can result in very rapid speciation 2 Basic modes: Auto- and Allo-Polyploidy Sympatric speciation by auto polyploidy in plants. The tetraploid offspring cannot backcross to the parent.
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One mechanism for allopolyploid speciation in plants. A hybrid of two different species is usually sterile because its chromosomes are not homologous and cannot pair during meiosis. However, such a hybrid may be able to reproduce asexually. This diagram traces one mechanism that can produce fertile hybrids (allopolyploids) as new species. The new species has a diploid chromosome number equal to the sum of the diploid chromosome numbers of the two parent species. Sympatric Speciation via Allo polyploidy Fig. 26.9
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Allotetraploids of Tragopogon (goatsbeards) These fowers are native to Eurasia. 2 new allopolyploid species were spontaneously generated in the northwestern U.S. by hybridization events involving 3 introduced diploids. Happened very quickly - within 50 years.
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Plant polyploids are very important in agriculture. Modern domesticated wheat is an allohexaploid (6 n ) produced by 2 hybridization events over the past 8,000 years in the Middle East.
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Testing Speciation When two formerly isolated populations return to sympatry, one of three things can be observed. If individuals from the formerly isolated populations begin interbreeding and the hybrids show no reduction in Ftness, the cohesiveness of the original species is returned. If the two populations remain reproductively isolated, a speciation event has occurred. In some cases, the populations may hybridize, and if the hybrids show a higher Ftness in an environment different from the parental species, they may persist as Hybrid Zone . Plains pocket gophers in eastern Nebraska have a dark pelage, which hides them from predators against the black soils there. Gophers in western Nebraska have lighter pelage, similar to the sandier soils there. In the area where the two soil types meet, a band of intermediate colored soil is home to hybrids. This is an example of a stable hybrid zone.
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Bio171-F11-Lec 27 - Biology 171 Wednesday, November 9, 2011...

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