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presentation_20 - 1 Hybrid zone exercise Nov 14 15 • Sign...

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Unformatted text preview: 1 Hybrid zone exercise Nov 14 - 15 • Sign up for hybrid zone exercise on birds at Laboratory of Ornithology (off campus) or on butterflies at Entomology Museum (Ent. Bldg) • Sign up sheets outside E137 Corson. Limited number of students per session. • Sessions Tu-Th at 12, 1, 2 & 3 pm. • Directions to Lab of Ornithology posted on web. Count on 1 hr 45 min to 2 hrs for travel time + exercise. • Handout for exercise also posted on web. Lecture 20 Models of Speciation 2 Fundamental issues in speciation • What is a species? • Geographic context. – Do speciation events require geographic isolation? – Allopatric speciation sympatric speciation. • Role of natural selection. – Are barriers to gene exchange "molded" by natural selection or are they simply a fortuitous consequence of divergence in allopatry? • Genetic architecture. – Number, effect (large or small), and chromosomal distribution of genes that are responsible for reproductive isolation. Current classification based on geography • Allopatric (m=0) (“different country”) • Parapatric (m>0, m<0.5) (“at the side of”) • Sympatric (m= 0.5) (“same”) • Really a continuum m 3 Geography of Speciation Allopatric Parapatric Sympatric P: Pacific species C: Carribean species Isthmus of Panama became a “complete” barrier about 3 mya. Snapping shrimp Vicariance in allopatric speciation Splitting of 2 large populations 4 Other allopatric models peripatric speciation: divergence & speciation in small isolated populations Refugia Fragmentation Colonization Founder event Side blotched lizard in on islands in the Gulf of Mexico G u l f o f C a l i f o r n i a 5 island mainland • Similarities- Natural selection (adaptation to different environments) or sexual selection (interactions between /within sexes) can lead to divergence in both cases- No selection: divergence due to prolonged and chance accumulation of different alleles (substitution). Processes responsible for allopatric divergence 6 Processes responsible for allopatric divergence • Difference between isolation of 2 large populations vs. small isolated population- Small isolated populations can go through genetic bottleneck (Mayr).- Contain less variation genetic.- Role of genetic drift in divergence & speciation larger in small population. • No role for selection: Differences between allopatric populations lead to pre- or post- zygotic barriers to gene exchange as a by- product ....
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This note was uploaded on 09/28/2008 for the course BIOEE 2780 taught by Professor Geber during the Fall '07 term at Cornell.

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presentation_20 - 1 Hybrid zone exercise Nov 14 15 • Sign...

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