EEB318-Cutter-Lecture5 - Genetic Drift 1 Allisons 2004...

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Genetic Drift 1
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Allison’s 2004 Perspective in Genetics Historical account of human sickle-cell disease research Mostly in Africa Simple genetic basis to malarial immunity Two red blood cell phenotypes Two main hemoglobin assay phenotypes HbA & HbS HbS dominant to HbA wrt presence of sickled cells AA homozygotes most common SS homozygotes highly detrimental AS heterozygotes beneficial for children infected with Plasmodium Allison 1954 0% 20% 40% 60% 80% 100% sicklers non-sicklers with parasitaemia without parasitaemia 27.9% 72.1% 45.7% 53.3% 2 Percent of patients
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Allison’s 2004 Perspective in Genetics Geographic distribution of Plasmodium and AS genotypes highly correlated 3
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Allison’s 2004 Perspective in Genetics World-wide, many HbS haplotypes Independent evolution of HbS (4 African + 1 south Asia) Same causal protein difference (glutamic acid G A G Æ valine G T G) Haplotype = unique combination of variant sites Lapoumeroulie et al. 1992 HbA haplotype HbS haplotypes GCTTC77CA ..... 85.. ..G..84T. AG..T84.C .G.C.69.. .G.C.95.. 4 A T T T T T
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Genetic Drift 5
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What does “chance” mean in evolution? Randomness in physical processes Two or more outcomes are possible… Can’t predict with certainty which outcome will manifest in any particular case Can predict what the possible outcomes are Can predict the probability of alternative outcomes Stochastic (unpredictable) evolutionary forces: Mutation Recombination Migration Genetic Drift Deterministic (predictable) evolutionary force: Natural Selection 6
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What does chance mean for evolution? Evolution = heritable change over time Changes in allele frequency in a population from one generation to the next Chance results in evolution that is non-adaptive NOT mal-adaptive or counter-adaptive Changes that occur independently of individuals’ ability to survive and reproduce Provides a null hypothesis for evolutionary change Compare observed pattern to that expected due simply to chance A test for the action of natural selection Do not assume adaptation by selection…need to prove it! 7
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Are stochastic processes sufficient to explain the characteristic? Comparison to null distribution Characteristic of interest Expected distribution due to chance processes 8
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Changes in allele frequency due to drift 9 What is the evolutionary fate of a new “neutral” mutation that does not affect fitness? a) It is lost from the population (goes extinct) b) It fixes in the population, becoming the only allele c) Persists forever, but is very rare d) Increases to 50% frequency, and stays at that frequency e) Its frequency fluctuates up and down at random forever
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Genetic drift is an inevitable consequence of “sampling error” The evolutionary fate of a new mutation Selectively neutral Fixation (frequency = 1) or loss (frequency = 0) Changes in allele frequency due to drift 10 p A1 = 1 p A1 = (2N-1)/(2N) = 1 1/(2N) q A2 = 1 - p A1 = 1/(2N)
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