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Molecular Evolution Course #27615. Anders Gorm Pedersen Molecular Evolution Group Center for Biological Sequence Analysis Technical University of Denmark (DTU) [email protected]. Neutral Theory of Molecular Evolution. Evolution is a two-step process: Mutation (random) Selection (non-random) - PowerPoint PPT Presentation
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Course #27615
Anders Gorm PedersenAnders Gorm Pedersen
Molecular Evolution GroupMolecular Evolution GroupCenter for Biological Sequence AnalysisCenter for Biological Sequence AnalysisTechnical University of Denmark (DTU)Technical University of Denmark (DTU)
[email protected]@cbs.dtu.dk
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Neutral Theory of Molecular Evolution
Evolution is a two-step process:Evolution is a two-step process:
1.1. Mutation (random)Mutation (random)2.2. Selection (non-random)Selection (non-random)
Detrimental mutation =>Detrimental mutation => negative selection =>negative selection => Mutation not seenMutation not seen
Beneficial mutation =>Beneficial mutation => positive selection =>positive selection => Mutation seenMutation seen
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Selectionist Views of What Drives Molecular Evolution
• Majority of all mutations are detrimental and not seenMajority of all mutations are detrimental and not seen• Most observed mutations have adaptive valueMost observed mutations have adaptive value
• Classical school:Classical school:– Single predominant version of gene (“wild type”) present in populationSingle predominant version of gene (“wild type”) present in population– Natural selection rapidly fixates new, advantageous mutationsNatural selection rapidly fixates new, advantageous mutations
• Balance school:Balance school:– Appreciable amount of polymorphism in gene poolAppreciable amount of polymorphism in gene pool– Polymorphism maintained actively by natural selection (e.g., sickle cell anemia) Polymorphism maintained actively by natural selection (e.g., sickle cell anemia)
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Neutralist Views of What Drives Molecular Evolution
• Electrophoretic studies in 1960’s showed much higher polymorphism than Electrophoretic studies in 1960’s showed much higher polymorphism than anticipated by either classical or balance school selectionistsanticipated by either classical or balance school selectionists
• Kimura and others proposed the “Neutral Theory of Molecular Evolution”.Kimura and others proposed the “Neutral Theory of Molecular Evolution”.
Detrimental mutation => negative selection => Mutation not seenDetrimental mutation => negative selection => Mutation not seenNeutral mutation => no selection => Mutation may be seen (genetic drift)Neutral mutation => no selection => Mutation may be seen (genetic drift)Beneficial mutation => positive selection => Mutation seenBeneficial mutation => positive selection => Mutation seen
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Difference Between Selectionist and Neutralist Views of Evolution
• Selectionist view:• Most observed mutations represent functional innovation
• Neutralist view:• Most observed mutations represent conservative changes, changes in unimportant regions
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All Agree that Adaptations are Caused by Natural Selection
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All Agree that Adaptations are Caused by Natural Selection
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All Agree that Adaptations are Caused by Natural Selection
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All Agree that Adaptations are Caused by Natural Selection
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All Agree that Adaptations are Caused by Natural Selection
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The molecular clock
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Genetic drift
Gen. 1Gen. 1
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Genetic drift
Gen. 1Gen. 1
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Genetic drift
Gen. 1Gen. 1
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Genetic drift
Gen. 1Gen. 1
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2 Gen. 3Gen. 3
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2 Gen. 3Gen. 3
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2 Gen. 3Gen. 3
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Genetic drift
Gen. 1Gen. 1 Gen. 2Gen. 2 Gen. 3Gen. 3 Gen. 4Gen. 4
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Genetic drift
Alleles will eventually reach a frequency of 0 or 1
Genetic diversity decreases
Effect is more strongly felt in small populations
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Time to fixation and time between fixations
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Drift and mutation
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Genetic Drift: The bottleneck effect
“Alleles” in original population
“Alleles” remaining after bottleneck
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CheetahBottleneck effect
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Northern Elephant Seal
• Reduced to 20 individuals in 1896• Now 30,000 individuals, with no detectable genetic diversity
BottleneckEffect
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Genetic Drift: The founder effect
• Change in allele frequencies when a new population arises from only a few individuals.
e.g., only a few fish are introduced into a lake.e.g., only a few birds make it to an island.
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ScorpaenidaeLionfishPterois volitans
FounderEffect
•New Atlantic population, maybe from only 10 individuals
www.fishbase.org
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Exercise: Genetic drift simulation
• Starting point: population with Starting point: population with NN individuals, fraction individuals, fraction pp has genotype has genotype A, A, fraction fraction (1-p)(1-p) has genotype has genotype aa
• All individuals produce 200 offspring of same genotype as parent. (offspring also has All individuals produce 200 offspring of same genotype as parent. (offspring also has fraction fraction pp genotype genotype AA))
• Survival rate = 1/200 => Constant population size Survival rate = 1/200 => Constant population size NN
• Death strikes randomly: each generation N random individuals surviveDeath strikes randomly: each generation N random individuals survive
• Investigate drift of allele Investigate drift of allele AA frequency:. frequency:. – Find proportion of populations where A is fixed (p=0.4; N=20, N=80)Find proportion of populations where A is fixed (p=0.4; N=20, N=80)– Find average time to fixation of A (p=0.4, N=10, 20, ...130) Find average time to fixation of A (p=0.4, N=10, 20, ...130)
