Flashcards in Lecture 7 Deck (19):
1
How do you simulate genetic drift?
1. Starting frequency is 0.2
2. Choose an allele at random, copy it and put it into the population of the next generation
3. Repeat until population size is reached
4. Repeat sampling through generations
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WrightFisher population:
 Each allele is equally likely to be sampled in the next generation
 Typically assumes a diploid and that self fertilisation is possible
 Binomial sampling
 Variance greatest at p=0..5
 Variance is greater in smaller pppulations
3
Genetic drift:
 In each population heterozygosity decays with time
 The starting frequency gives you the probability of fixation
 Very weak/slow in large populations
 Gene flow counteracts fixation
 Eliminates variation
4
Decay in heterozygosity by drift:
H=1G, where
H = frequency of the heterozygotes in the population
G = frequency of the homozygotes in a population
Ht = Ho(11/2N)to the power of t
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Ht = Ho(11/2N)to the power of t
 Ht is expected heterozygosity at time t
 Ho: initial heterozygosity
 N = population size
 The decay in any single population will be much more stochastic
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If N is really high (eg, 1,000, 000)
 t0.5=1.38x10to the power of 6 generations (the time it takes to lose half the heterozygosity via drift)..
 This means that it will take around 28 million years to loose half the heterozygosity in humans (with a generation time of 20 years.
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Mutation (u) and drift:
 H=4Ni/(1+4Nu)
 Heterozygosity in the population is determined by the mutation rate and the population size
 This is the equilibrium value of heterozygosity
8
A problem in Neutral Theory:
 The generation time effect
 Expected and Observed are so different
 Expect: many organisms have extreme levels of heterozygosity
 Observe: correlation between population size and heterozygosity is weak
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Effective population size (Ne)
 The size of the idealised (wrightFisher) population whose decay of heterozygosity equals that of the real populations
 May be smaller than the breeding population
 Allows us to accommodate the dramatic fluctuations of population sizes over time
 The harmonic mean of the population sizes
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Harmonic mean:
 Dominated by the smallest numbers in the sample
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Factors effecting Ne:
 Different number of males than females
 Y chromosome
 Mitochondria

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Different number of males than females: Ne = 4NmNf/Nm+Nf
eg) Elephant seals have harems, 5 males and 20 females (ie. Nc=25, Ne = 16)
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Ne for Y: Ne=Nem/2
 Y is only in males, and only one copy per male
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Ne for mitochondria: Ne(y)=Ne(mitochondria)
 Mitochondria is in al individuals but is almost always passed from mothers to children, and mothers almost always have a single version.
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The X chromosome or haplodiploidy: Ne=9NmNf/4Nm+2nf
eg) a bee hive may have 20,000 bess, a singel queen and a dozen drones, Ne=9x12/4x12+2 = 2.16
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Migration:
 Individuals that move from one area to a new area
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Gene flow:
 To move to a new area and interbreed with resident populations
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ContinentIsland model is one way flow from a very large stable population:
 Pc m> Pi
 Pc is the frequency on the continent, last generation migrants
 Pi is the island, last generation residents
 m is the proportion of alleles that has come from another population in that generation
 Eventually, because the population is finite, they will all fix for one allele.
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