presentation_19 - Lecture 19 What are species? Questions 1....

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1 Lecture 19 What are species? 1. What is a species? 2. What are the roles of selection, drift and mutation in divergence and speciation? 4. What genetic changes are associated with speciation? Questions
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2 Anagenesis Gradual change in phenotype over time Phenotype Time anagenesis round & long Sp. A Sp. A’? anagenesis Trilobites
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3 ANAGENESIS Evolution of “beak” length in the Soapberry bug Balloon vine 1 (native) Florida Golden rain tree (introduced) Soapberry bug Work by S. Carroll et al. DATE 9 8 7 6 Beak length of bugs in Florida introduction of golden rain tree to Florida Beak length (mm) Cladogenesis Splitting of lineages Phenotype Time anagenesis cladogenesis square & long round & long Sp. A Sp. A’? Sp. B
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4 Debate on what is a species? Speciation and divergence are part of an evolutionary process …at any time expect to Fnd lineages at all stages of divergence. t 2 t 1
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5 “ring” species • “. ..groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” (Mayr) • “. ..systems of populations; the gene exchange between these systems is limited or prevented in nature by a reproductive isolating mechanism or by a combination of such mechanisms.” (Dobzhansky) Biological species concept (“isolation concept”)
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6 Some real problems with the BSC • DifFcult to actually apply: – requires that we determine patterns of gene exchange – hybridization – cannot be applied to allopatric populations – does not apply to asexual lineages. • Reproductive isolation may be incomplete (some level of hybridization and introgression). That is, barriers to gene exchange are not necessarily all or none. Hybridization One species or two?? Red-shafted (western) and yellow-shafted (eastern) intergrade broadly in the Great Plains.
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7 Allopatric species Liriodendron (tulip tree) found in eastern NA ( L. tulipifera ) and eastern Asia ( L. chinense ) © 2002 Steve Baskauf 65 genera show the same disjunct distribution. Many congeneric pairs appear to have diverged relatively recently. Are they the same species or different species??
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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 University (Engineering School).

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presentation_19 - Lecture 19 What are species? Questions 1....

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