migration, drift, and non-random matingdgray/evol322/chapter7.pdf · • but found at 0.07 in...
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Migration, drift, and non-randommating
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Hardy-Weinberg conditions
• No mutation
• No selection
• No migration
• No genetic drift
• No non-random mating
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If Hardy-Weinberg holds, then…
• No allele frequency changep = frequency of allele A
q = frequency of allele a
• Genotype frequencies follow fromp2 + 2pq + q2
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Migration
• Not seasonal movement– E.g. birds
• Movement of alleles form one population toanother– Called ‘gene flow’
• Makes populations more similar to eachother
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Migration
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Nerodia sipedon
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Selection on banding pattern
• Mainland– Banded snakes favored (dappled light)
• Islands– Unbanded snakes favored
• Barren limestone basking surfaces
• Banded alleles on island persist due tomigration from mainland
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Migration of alleles
• Changes allele frequencies
• Can alter genotype frequencies
• Makes populations more similar
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Measuring genetic similarity ofpopulations
• Fst statistic ranges from 0 to 1
• Measures variation among subpopulationsrelative to the total variation (s and t)
• Fst high, then subpopulations pretty distinct
• Fst low, subpopulations homogenous
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Silene dioica Swedish islands
• Colonize young island– Genes that get to any specific island mostly a matter
of chance
• Pollination by insects– Over time, genes get spread from island to island
(migration of alleles)
• Die off through ecological succession– Old populations survivors stochastic
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Giles and Goulet, 1997
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Genetic drift
• In Giles and Goulet’s study, what accountsfor the high Fst values for youngpopulations?
• Chance founder events– Populations drawn from small potential pool
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Population size and genetic drift
• Flip a coin, odds are even (50:50) heads ortails
• If you flip the coin 10, 000, 000 times– You’d better get really close to 50:50
• If you flip the coin only 4 times, you have agood chance of getting either all heads or alltails12.5% chance, even if the coin is a fair coin
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Sampling error in small populations
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Chance of random allele frequencychange, N = 10 zygotes
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Drift versus sample size
• 3 runs of a simulationmodel
• True allele frequency60:40
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Drift as an evolutionary force
• Drift not an important evolutionary force inlarge populations
• Can be important in small populations– Founding of new populations
– Fixation of alleles, loss of heterozygosity
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Founder effect
• High Fst in Silene dioica young populations
• In humans,– Ellis-van-Creveld syndrome
• Rare form of Dwarfism
• Allele frequency around 0.001 in most populations
• But found at 0.07 in Pennsylvania Amish descendedfrom 200 founding individuals
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Drift and allele frequency change
• small populations overmany generations
• Fixation: an allele is fixedat a locus if it is at afrequency of 100%
• Heterozygosity decreasesas alleles becomes rarer Note: 2(p)*(1-p)
= 2pq
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Fixation of alleles
• If allele frequency goes to 1 it is fixed
• If it goes to 0 the allele is lost, and thealternative allele is fixed (if there are onlytwo alleles)
• Probability that an allele goes to fixationequal to its initial frequency– With drift alone that is (no mutation, no
selection, etc.)
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Loss of heterozygosity• Heterozygote frequency = 2 pq
– Alternatively 2p(1-p)– At a maximum when p = 0.5
• Buri Drosophila experiment• 107 lines of 8 females 8 males• Start p = q = 0.5• Qualitative: heterozygosity
decrease• Quantitative: for population with
size 16, heterozygosity shouldfollow dashed line; insteadfollowed solid gray line - theprediction for n = 9
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Effective population size• Buri’s fly populations lost heterozygosity as
predicted IF the population size was 9 not 16
• If some died, or failed to reproduce, then theeffective population size can be smaller than theactual population size
• Ne = (4 NmNf)/(Nm + Nf)Nm = number of sexually reproductive males
Nf = number of sexually reproducing females
• 5 males 5 females, Ne = 10
• 1 male 9 females, Ne = 3.6
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Drift and the neutral theory
• Alleles that have no fitness effect calledneutral
• Allelic substitution can be by drift orselection
• If most mutations produce selectively neutralalleles, the fate of those alleles will begoverned mostly by drift– Basis of idea behind molecular clock
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Genetic drift summary
• Random effects
• Importance highly dependent on populationsize– Effective population size even smaller
• Can allow a neutral allele to replace anothersimply by chance
• Decreases allelic diversity and heterozygosity
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Non-random mating
• Obviously individuals do not mate randomly– Really, would you want to mate randomly?
• We are talking about random mating withrespect to particular alleles
• Not non-random mating with respect to money,sexiness, or ability to make your heart go pitter-patter– That is sexual selection, a form of natural selection
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Non-random mating with respectto alleles
• Positive assortative mating– Like mates with like
• Mating among genetic relatives calledInbreeding
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Inbreeding and heterozygosity
• Imagine extreme inbreeding
• Self fertilization
• Homozygotes produce all homozygotes
• Heterozygotes produce 1/2 homozygotesand 1/2 heterozygotes
• Proportion of heterozygotes decreases by1/2 each generation
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Selfing and heterozygosity
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Inbreeding produces excesshomozygotes
• More homozygotes than predicted byHardy-Weinberg suggests something,perhaps inbreeding is going on
• One generation of random mating re-establishes Hardy-Weinberg genotypefrequencies
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Inbreeding depression
• Does not mean you are sad you kissed yourcousin
• Inbreeding produces a deficit ofheterozygotes and a surplus of homozygotes
• What if those homozygotes are ofdeleterious recessive alleles?
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Inbreeding reduces fitness: humans
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Also, plants, non-human animals
Blue outcrossed controls; red selfed
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Conservation Genetics:the case of the greater prairie
chicken in Illinois
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Movie time
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Decline
• Millions pre-1837 steel plow
• 25000 in 1933
• 2000 in 1962
• 500 in 1972
• 76 in 1990
• 50 or less 1994
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Habitat loss: steel plow 1837
Two remaining habitats protected in 1962 and 1967
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Protection and population decline
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Why the post mid 1970’s decline?
• Migration
• Drift
• Inbreeding
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Allelic diversity
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Egg viability
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Evolutionary forces
• Drift– Small population
– Even smaller effective population size• Lek mating system
• Low allelic diversity, low heterozygosity
• Migration reintroduces new alleles– Gene flow