Lect9_drift

Lect9_drift - Causes of Evolution: Drift versus Selection +...

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1 Causes of Evolution: Drift versus Selection + Ice Fishes case study Day #9 Today’s 3 goals … Be able to answer these questions • 1. How can drift cause a gene or trait to increase in frequency? • 2. What types of changes are observed in actual genes? • 3. Why are many of the changes observed neutral? Errors are Inevitable: Mutations cause DNA (and amino acid) sequences to diverge over time Sequence X Sequence A Sequence B Genetic drift occurs when selection cannot detect or cannot remove mutations Genetic Drift? • Any change in base sequence is detectable using modern genetic techniques, but many do not affect phenotype • Selection acts on phenotype • Thus, some genetic changes are invisible to selection – Silent mutations (usually undetectable, with some exceptions) – Some missense mutations (amino acids with similar chemistries) • Other genetic changes persist despite reducing fitness – Selection is more powerful in large populations You might expect the following: Frequency of new allele “a 1 of GeneA 1.0 0 Freq is low when a 1 first forms in population of “a” (freq is a 1 /(a + a 1 ) Generations after formation of a 1 If a 1 improves reproductive success If a 1 reduces reproductive success (loss and reoccurrence) If a1 has no effect on reproduction it i ncreases slowly due to the accumulation of new mutations Actually, this is what you see if a 1 is beneficial : Frequency of new allele “a a of GeneA 1.0 0 Freq is low when a 1 first forms Time (mutations forming repeatedly) Many new mutations get lost. If the mutation manages to get common enough, it ‘escapes’ drift and can sweep the population Drift wins. Beneficial mutation is lost. Selection wins
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2 And if new a 1 is neutral, you see this: Frequency of new allele “a a of GeneA 1.0 0 Freq is low when a 1 first forms Time (mutations forming repeatedly) Most alleles are lost ….but they can hang around a long time Random changes in allele frequency - (drift) Without selection, one allele will eventually “win” by chance And if new a 1 is neutral, you see this: Frequency of new allele “a a of GeneA 1.0 0 Freq is low when a 1 first forms Time (mutations forming repeatedly) Random changes in allele frequency - (drift) If each new neutral allele has the same probability as any to sweep the population, then Probability of winning = 1/pop size NOTICE POPULATION SIZE EFFECT If you start a population with a 1:1 mix of two neutral alleles 2500 250 0.5 1.0 Freq. of a 1 25 individuals A particular Allele will drift either to fixation or be lost, It takes longer in large population (affects of chance events and thus fluctuations in allele number are smaller) One or the other will ultimately win DRIFT IS A STRONGER FORCE IN SMALL POPULATIONS Observation: Proteins with similar sequences exist in many organisms • Gene name: Amino Acid sequence • Aniridia (human): LQRNRTSFT QE QI EA LEKEFERTHYPDVFARERLA A KI D LPEARIQVWFSNRRAKWRREE • Eyeless ( Drosophila
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Lect9_drift - Causes of Evolution: Drift versus Selection +...

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