1Causes of Evolution: Drift versus Selection+Ice Fishes case studyDay #9Today’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 timeSequence XSequence ASequence BGenetic driftoccurs when selection cannot detect or cannot remove mutationsGenetic 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 populationsYou might expect the following:Frequencyof newallele “a1”of GeneA1.00Freq is lowwhen a1first formsin population of “a”(freq is a1/(a + a1)Generations after formation of a1If a1 improvesreproductivesuccessIf a1 reduces reproductivesuccess (loss and reoccurrence)If a1 has no effecton reproduction itincreases slowlydue to the accumulation of new mutationsActually, this is what you seeif a1 is beneficial:Frequencyof newallele “aa”of GeneA1.00Freq is lowwhen a1first formsTime (mutations forming repeatedly) Many new mutations get lost.If the mutation manages to get common enough, it‘escapes’ drift and can sweep the populationDrift wins.Beneficial mutation islost.Selection wins
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