Lecture - Speciation

Lecture - Speciation - Speciation BarrierstoGeneFlow

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Speciation 
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Barriers to Gene Flow Whether or not a physical barrier  deters gene flow depends upon: Organism’s mode of dispersal or  locomotion Duration of time organism can move 
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Genetic Drift in  Snail Populations Robert Selander studied  Helix  aspersa    Collected snails from a two-block  area Analyzed the allele frequencies for  five genes
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Genetic Divergence in  Snail Populations
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Snail Speciation? Will the time come when the snails from  opposite sides of the street are so different  that they can no longer interbreed? If so, then they will have become two  distinct species
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Natural selection  can  lead to speciation Speciation can also occur as a result of  other microevolutionary processes Genetic drift Mutation Sexual Selection
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Morphological traits may not be useful in  distinguishing species Members of same species may appear  different because of environmental  conditions Morphology can vary with age and sex Different species can appear identical
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Darwin's Explanatory Model of Evolution  Through Natural Selection Refer to Excel File
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Variable Morphology Grown in water Grown on land
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Biological Species Concept  “Species are groups of interbreeding natural  populations that are reproductively isolated  from other  such groups.” Ernst Mayr
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Reproductive Isolation Cornerstone of the biological species  concept Speciation is the attainment of reproductive  isolation Reproductive isolation arises as a  by-product of genetic change
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Genetic Divergence Gradual accumulation of differences in the  gene pools of populations Natural selection, genetic drift, and  mutation can contribute to divergence Gene flow counters divergence
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Genetic Divergence time A time B time C time D daughter species parent species
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Reproductive Isolating Mechanisms Prezygotic isolation Mating or zygote formation is prevented Postzygotic isolation Takes effect after hybrid zygotes form Zygotes may die early, be weak, or be  sterile
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Prezygotic Isolation Ecological Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation  Gametic Mortality
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Ecological Isolation and Competitive Exclusion in Two  Crayfish ( Orconectes Virilis  and  Orconectes Immunis ) Ecology:  Vol. 51, No. 2, pp. 225–236. Richard V. Bovbjerg
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Lecture - Speciation - Speciation BarrierstoGeneFlow

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