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gb640Chp19 - POPULATION STRUCTURE Concepts of population...

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Concepts of population structure POPULATION STRUCTURE HOW IS GENETIC VARIATION DISTRIBUTED INTO DIFFERENT REGIONS OF THE SPECIES IS VARIATION HOMOGENEOUS? ARE DIFFERENT POPULATIONS REALLY DIFFERENT? Process of genetic differentiation acquisition of allele frequencies that differ among subpopulations may result from several factors • selection acting differently in different parts of the species range • chance effects – random drift – founder effect • Unique mutations - occurred in one area and now spreading hierarchical population structure Analytical method to partition genetic variability occurring in a “population” – How much is unique to a single local population – How much is unique to local clusters of populations (local races) – How much is unique to regional clusters of populations (races or subspecies) – How much is shared by all members of the species consider a freshwater fish Meffe & Carroll, 1994 Meffe & Carroll, 1994 differences between populations in same stream
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Meffe & Carroll, 1994 Meffe & Carroll, 1994 differences between populations in adjacent streams Meffe & Carroll, 1994 differences between streams in same river Meffe & Carroll, 1994 differences between areas in watersheds Meffe & Carroll, 1994 differences between watersheds Meffe & Carroll, 1994 total variation example of hierarchy Fundulus heteroclitIs – saltmarsh killifish – range - Atlantic coastal rivers and saltmarshes of U.S. / Canada Nova Scotia - Georgia Do all the local populations make up a single “random mating” POPULATION?
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Geographic variation in Fundulus heteroclitis along Atlantic coast North versus South Within South Subgroups Within South effects of population subdivision on genetic variation Subdivision reduces the observed heterozygosity – compared to levels expected under random mating • same type of effect as seen with inbreeding reduction in average heterozygosity due to population structure RANDOM MATING RANDOM MATING
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reduction in average heterozygosity due to population structure TOTAL ALLELE FREQUENCY = 0.5 reduction in average heterozygosity due to population structure HETEROZYGOSITY = 0 EXPECTATIONS With random mating: – 2 p q = 2 x 0.5 x 0.5 = 0.5 Observed = 0.0 Concepts of population structure population B
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