If there is no selection then d n d s ratios among

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If there is no selection— then d N / d S ratios among closely related species should the same as the P N / p S within either of the species The MK Test 20
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– Differences between d N / d S ratios among closely related species and the P N / p S within either of the species indicates a the site under selection • d N / d S ratios > P N / p S ratios indicates positive selection • d N / d S ratios < P N / p S ratios indicates negative (“purifying”) selection • d N / d S ratios = P N / p S ratios indicates no selection The MK Test 21
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All new mutations start out as polymorphisms synonymous subs change by drift alone and will remain as polymorphisms closely related species will exhibit similar numbers of these advantageous nonsynonymous subs will become fixed and will no longer be a polymorphism these are the very changes that contribute to species differences The MK Test 22
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Adh gene—produces protein that neutralizes toxic alcohol This gene is likely under selection alcohol breakdown is important to fruit flies different food sources produce different amounts of alcohol 30 individuals from three separate species from very separate locations Wild Fruit Flies 23
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Looked at Adh gene sequence in each individual Counted the number of fixed vs polymorphic differences in each fly’s Adh sequence fixed = present in all members of one species but different from other species polymorphic = present in only some members of the species The Original MK Test—Fruit Flies 24
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Species 1 ACCT G CCA ACCT G CCA Polymorphic T CCT G CCA AC G T G CCA Fixed Species 2 ACCT C CCA ACCT C C G A ACCT C CCA ACC A C CCA Fixed vs Polymorphic 25
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What they found: 29% of the differences that were fixed between species were nonsynonymous only 5% of the polymorphisms within species were nonsynonymous This result is quite significant—there are nearly 6 times more replacements between species than within any one species this suggests that positive selection on advantageous change is far outpacing drift acting on neutral substitutions within this locus The Original MK Test 26
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Hughes and Nei (1988) and Hutley et al (2000) showed that at least some gene sequences are dominated by positive selection The MK test has revealed many more loci in which positive selection also outperforms genetic drift What the MK Test Reveals 27
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