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Consider a grid of small populations

Consider a grid of small populations - 0.5 then at the end...

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Consider a grid of small populations (e.g., ponds in Minnesota), all with the same small population size and all starting at time t with p = q= 0.5. Through time each population will experience genetic drift due to random sampling and the frequencies in each population will diverge. The distribution of frequencies changes over time from a tight distribution (all 0.5), to a flat distribution (some populations at p = 0.1, some at 0.9 and all frequencies in between), to fixation (p =1.0) or loss (p = 0.0) of the alleles in all populations (see figure below). Fixation is when all alleles in the population are A; this necessarily implies loss of the a allele ("fixation" or "loss" should only be used with reference to a specific allele). If each population starts at p =
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Unformatted text preview: 0.5, then at the end, when all populations have lost their variation, 50% of the populations will be fixed for the A allele and 50% will be fixed for the a allele (latter = "loss" for the A allele, get it?). If the initial frequency was p = 0.7, then 70% of the populations would be fixed for the A allele (again, assuming no selection, migration, mutation). Main Points: 1) total variation does not change; variation goes from within populations (no variation between populations) to between populations (no variation within populations). 2) genetic divergence of populations entirely by chance! (no selection). This is why genetic drift can be an important force in evolution....
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