14_Population_subdivision(1).pdf - FRSC-BIOL 3700 14 – Population subdivision Aaron Shafer Last Lecture ● Calculating inbreeding Last Lecture ●

14_Population_subdivision(1).pdf - FRSC-BIOL 3700 14 –...

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FRSC-BIOL 3700 14 – Population subdivision Aaron Shafer
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Last Lecture Calculating inbreeding
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Last Lecture Impact on genotype frequencies AA = p 2 + pFq Aa = 2pq(1 -F) aa = q 2 + qFp F ROH and F h
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Population subdivision Many species occupy such vast geographic areas or have such effective barriers to migration that they cannot behave as a single, randomly mating population In such cases, there will be genetic differentiation between subpopulations , which leads to departures from Hardy- Weinberg for the entire species.
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Population subdivision - humans
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Population subdivision - giraffes
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Population subdivision - mockingbirds
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Population subdivision In the case where there is random mating within each subdivision, the genotype frequencies for the entire species are described by a new incarnation of F called F ST The deviation from random mating that is F ST reflects the difference between the subpopulation (S) and the total population (T). This is an extension of last lectures inbreeding formulas, notably hierarchical F statistics
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Two populations HWE HWE Genotype frequency for entire species is simply the average of the two patches: (1 /16 + 9/16) / 2 = 5 / 16
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Two populations HWE HWE Expected genotype frequency of species under HWE; important to note that while each subpopulatin is meeting HW expectation, the entire species is not (because of subdivision!) In this case there are too many homozygotes.
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Two populations The excess of homozygotes requires an F ST of 0.25 (or 1/4). How do we know that? Let’s revisit our equation from last lecture: 2pq(1- F ) = 3/8 Rearrange and solve: (1 – F )/2 =3/8 = 1/4 BUT we have no idea if this excess in heterozygotes is due to inbreeding OR subdivision OR something else!
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