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Molecular evolution What is molecular evolution? Mutation, drift and selection acting at the molecular level. Change in DNA sequences. ATGGTCAAGCTTACCATG. ATG C TCAAGCTTACCATG. ATGGTCAAGCTTACCATG. ATGGTCAAG A TTACCATG. ATGGTCAAG A TTACC T TG. Molecular evolution
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Molecular evolution What is molecular evolution? Mutation, drift and selection acting at the molecular level Change in DNA sequences ATGGTCAAGCTTACCATG ATGCTCAAGCTTACCATG ATGGTCAAGCTTACCATG ATGGTCAAGATTACCATG ATGGTCAAGATTACCTTG
Molecular evolution • Mutation, drift and selection acting at the molecular level • Mutation – most mutations are base substitutions • Drift – most populations are finite, so drift can have some effect • some new mutations should drift out of population • some new mutations should eventually drift to fixation • Selection – acts on mutations that affect fitness • should act against harmful mutations elimination • should favor beneficial mutations fixation substitution – fixation of a new mutation ATGGTCAAGCTTACCATG ATGCTCAAGCTTACCATG ATGGTCAAGCTTACCATG ATGGTCAAGATTACCATG ATGGTCAAGATTACCTTG ATGGTCAAGATTACCATG ATGGTCAAGATTACCATC
Neutral Theory of Molecular Evolution remember....
Evolutionary biology since the modern synthesis • Revising the modern synthesis • Is evolution always gradual? • Eldredge and Gould, 1972, Punctuated Equilibrium • How important is genetic drift relative to natural selection? • Kimura, 1968, Neutral Theory • Is speciation always slow? • Does speciation only occur in isolated populations? LECTURE 12
How genetically variable are populations? H0: little variation LECTURE 15 - because selection should remove less fit alleles - implies: mutations either good or bad, not neutral Ha: lots of variation neutral theory – most mutations are neutral selectionist theory – variation in selection maintains different alleles in population - heterozygote advantage - fluctuating selection (selection varies in time or space) - frequency-dependent selection (rare allele is advantageous)
Neutral Theory of Molecular Evolution • most base substitutions are selectively neutral • drift dominates evolution at the molecular level • Under drift, rate of fixation should be steady through time • because drift is the result of chance alone (can happen any time) • predicts steady change through time = “molecular clock” ATGGTCAAGCTTACCATG ATGCTCAAGCTTACCATG ATGGTCAAGCTTACCATG ATGGTCAAGATTACCATG ATGGTCAAGATTACCTTG ATGGTCAAGATTACCATG ATGGTCAAGATTACCATC