OEB 167 2-27.docx

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Department
Organismic and Evolutionary Biology
Course Code
Organismic and Evolutionary Biology OEB 167
Professor
Jonathan Losos

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Variety and Placentation in Squamate Reptiles Viviparity Why is it so successful? What are selective pressures leading to evolution? Do they all do it in the same way, or do they have different mechanisms? What functional  mechanisms might we want to get at? How do pregnant females sustain embryos? Gas exchange, water, nutrients How do placentas develop and function? Selective pressures and constraints? Patterns of evolutionary change Reptile viviparity was long seen as simple “not really viviparous” “lack true placenta” unlikely to be relevant to mammal evolution viviparity has evolved over 90 times independently. Now, up to 115 separate origins of this pattern  (115 squamata, 6 extinct reptile, 8 lissamphibia, 23 fishes, 1 mammals). Distributed worldwide.  Actual number undoubtedly higher.   Sometimes, within the same species, if different populations, one population may be oviparous  instead of viviparous, etc.  >1800 extant squamate species viviparous (~20%), arisen >115 times in squamates, most origins  at low taxonomic level What factors affect evolution? Benefits Protect eggs from environmental hazard and predation Permits use of
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